I like Nicholas's response as-is, but will chime in here with a book recommendation: Robert Solomon's book About Love is an absolutely fantastic work. It is written to be accessed by anyone interested in love, marriage, or relationships. It is wonderfully clear and has been the intellectual highlight of my summer. I think it would help you thinking through your decisions ahead. Good luck.
"Fulfill" is a bit of a weasel word, isn't it? Suppose one partner would like to make love every night. The other, less libidinous spouse is more a two-or-three time a week type. We might say that the first spouse is "unfulfilled," but that sounds like a really poor excuse for adultery.
If the lack of "fulfillment" amount to some deep incompatibility, a good question to ask first might be: have the partners in the marriage talked about what's not working? Can it be fixed? If the answer really seems to be no, then the next obvious question is whether the marriage is worth saving.
Life is complicated, of course and blanket generalizations don't do justice to the complexity of people's relationships. But the old question: "How would I feel if the tables were turned?" is always a good one to ask when we're trying to decide if we're acting rightly. It's not just an old bromide; it gets at something pretty deep in our notions of right and wrong.
The texts of intimate relationships are generally too complicated to make judgments about using simple moral principles. But as a weakly stated general rule, I'd say that it's not wrong to marry or simply remain in a marriage out of a sense of duty. In fact, I would say that a sense of duty is a desirable element of a good foundation for marriage. It is, however, wrong to marry or remain married for the sake of duty but do so deceptively--that is, it is wrong to marry or stay married only or principally for sake of duty when your partner in marriage believes otherwise.
I can't imagine why you would think this is wrong. We're talking about consenting adults, right? If the relationship turns out to be very important to both of you, I would expect you would find ways to get back together again. Long distance relationships can be difficult, but they're not impossible!
How clear are we about the very idea of "falling in love"? When we talk about different cases of falling in love are we always talking about the same single kind of experience? Or are we perhaps talking about some quite complex pattern or syndrome of thoughts and feelings, which might come in different mixes in different cases (so that different instances of falling in love have a family resemblance to each other, though there perhaps needn't be any one core experience in common to all cases)?
Well, even if one person's own experiences might have a common thread, the anecdotes of friends and relations and the witness of however many novelists and poets do suggest that the experiences people call "falling in love" are indeed actually rather complex and many-stranded. And as a matter of fact, it seems that at least some of the components of these complex experiences come in degrees (so "falling in love" isn't really an all-or-nothing business).
Of course, noting this is, in part, to make a general empirical claim about the human psychology of pair-bonding rather than to advance a purely philosophical conceptual claim. But then it seems that the original question is, indeed, in part an empirical one, not simply to be settled by armchair reflection.
An interesting question. Of course, our neuroscientist has learnt something about love, for she has learnt something about the neural causes of certain feelings bound up with love. But you might well feel that there is a sense in which her discoveries don't help us understand what really matters about love as part of human life (hasn't in the important sense learnt about the nature of love). That needs a quite different sort of enquiry, pursued by poets and playwrights and novelists down the ages. Compare: someone who tells us about the chemical composition of the pigments used in Botticelli's Primavera has told us something about the painting. But again such discoveries don't help us understand the painting in the way that matters, as a work of art, as part of the human world: understanding that requires something quite different from chemistry.
We could stop there. But perhaps there is a bit more that needs to be said. For there can remain a nagging feeling that the neuroscientist has in some sense diminished love, shown that it is "just chemistry". Yet is that right? Must finding out about various causes and correlates of mental states in some sense undermine them, unmask them as not what we thought them to be? Well, let's consider various other cases before turning back to love.
Start with beliefs. What causes me to believe that there's a computer screen in front of me right now? No doubt there is a long and complicated physical story to be told -- light emitted by the screen, affecting my eyes, rods and cones in the retina doing their stuff, signals going up the optic nerve, etc. etc. All very interesting -- and of course not at all worrying! To be told that such a belief is produced by a lot of causal processes of that kind doesn't in any way undermine the belief. On the contrary: I postively want my perceptual beliefs about the world to be caused by the appropriate functioning of my sensory apparatus as reliable generators of true beliefs.
But other beliefs might be caused in less desirable ways. Jack's religious beliefs, say, may be causally grounded in stories prevalent in the community he was brought up in, and be causally sustained e.g. by the emotional comfort they bring him in bonding him to that community. And when he comes to realize that, then this fact might indeed be worrying: for he might well think, on reflection, that those kinds of causal processes aren't particularly liable to produce true beliefs (given that the same sort of processes functioning in other religious communities produce quite different beliefs). In this case, the causal explanation might well be regarded us "unmasking" the belief, revealing it to be not in good order.
What about desires? Why do I desire chocolate, say. One account has it that there are chemicals in chocolate that in a mild way act rather like THC (the active ingredient of cannabis). Interesting if true. No wonder I like chocolate! And coming to believe this account which explains my desire doesn't do anything to diminish my desire, or unmask it as inappropriate.
With other desires, however, it can be different. Coming to recognize the cause of a desire can diminish it. If it dawns on me that I want SuperDuperExpensive Corn Flakes rather than ValueOwnBrand Flakes, not because they are better, but only because I've been manipulated by clever advertising, then my preferences have been "unmasked" and may well change as a consequence.
So like the belief case, coming to discover the causes of desires can leave them in place in certain cases, but might be undermining in other cases.
And now what about romantic love? If Mercutio whispers in Romeo's ear, "It's the serotonin, old chap", will that change his feelings for Juliet? Has his love been rudely unmasked, e.g. as just a desire for cheap chemical thrills?
I don't suppose Romeo is much in the mood to be distracted by such thoughts. But, waiting for Juliet's household to get to bed so he can climb up to her balcony, he might reflect how interesting the chemistry of love must be (and one day, when he has less pressing business to attend to, he must learn more about it). He also recalls his school-room reading of Book One of Plato's Republic, and the old man Cephalus calming accepting that "the pleasures of youth and love are fled away". But Romeo is only too glad that he is young, his chemical systems are bursting with vim and vigour, and his brain still gets awash with serotonin at the sight of a pretty girl. He is very happy, so to speak, to go with the chemical flow. So Romeo's feelings for Juliet aren't changed in the slightest by reflecting on their neural causes any more than my belief that there is a screen in front of me and my desire for chocolate are changed by reflecting on their causes. And he'll think that the fact that his feelings have a "chemical composition" no more shows that they are just chemistry (in any important sense) than our scientist showed that Primavera is just a load of old chemicals! His feelings have a role and place in his life that he values, and it is that which matters about them.
I'm with Romeo on this.
You seem to be beginning with the assumption that romantic love must be essentially unselfish, that people must be motivated to love their lovers for the sake of their lover (only?) and not themselves (at all?). I'm not sure why we should think that's true.
Perhaps it seems that way because we normally assume that people are and should be willing to sacrifice their own interests for the people they love. And that surely is true. But those unselfish feelings and behaviors that occur within the relationship are (a) entirely consistent with falling in love for more selfish reasons, some you are aware of--the other person makes you feel good in lots of way--and others you aren't aware of--your genes dispose the development of your brain so that it picks up on features of your lover that make them appear attractive to you (there's all those studies on pheremones), and (b) entirely consistent with your continuing to get satisfaction from your relationship and that being a significant motivation for continuing in it.
You seem to be pointing to a common confusion in the debate about whether we are only nice to others for ultimately selfish reasons. But if making others happy, helping them, even sacrificing your own material goods and time for them, makes you feel good and if part of your motivation is that good feeling, I think it is still accurate to say that you are being altruistic (and loving) rather than selfish.
The tricky case is what happens if, for whatever reason, you lose the good feelings. Will you still be good to your lover? Do you still love him/her? Perhaps there is a deep sort of committed love that endures in these cases. But perhaps it is no longer romantic love?
I have always thought it meant that I used to love you but I still have vestiges of my earlier feelings for you, which are now thoroughly diminished. It sort of acknowledges that once one has been in love with someone it is difficult to reduce the relationship to mere friendship (much easier to change it to hate!).
You assume (1) that you can, to a large extent anyway, choose whether or not to let yourself fall in love with someone new versus sustain the love you already feel for the person you are involved with, and (2) that you have no reason to think the new relationship will be any happier than the old (although you also claim to think that both you and your past girlfriend are better off now than you were before). You also seem to assume (3) that future happiness of your new acquaintance would be the same whether or not you allowed yourself to fall in love with her (presumably because without you she would be involved in some other, equally happy relationship). It is usually very hard to know the accuracy of each of these assumptions, but I do not find them unreasonable.
On a straightforward comparison of current happiness with probable future happiness of everyone involved, there seems to be no reason to choose one relationship over the other. You seem to suggest, however, that the transition itself would add to your happiness (because of the special excitement of something new?) while it would detract from the happiness of the person you were first involved with. Instead of giving priority to either your happiness or hers, you could try to figure out whether her lost happiness is greater than your gained happiness, and then act so as to bring about the greatest total happiness. Again, this is something that it is usually very hard to know -- both because of the difficulty of predicting happiness in general, but also because of the difficulty of measuring different sorts of happiness and unhappiness on a single scale. Still, there are certainly situations in which you can know that someone else's loss of happiness is greater than your gain, and that seems to be the situation that you were in.
Your difficulty is not simply the difficulty of deciding whose happiness matters more, however. There are many ethical considerations that depend on something other than the resulting happiness of the people involved. First, there are obligations that we incur by agreeing to certain rules or contracts -- by making promises, for example. If you make a promise to remain devoted to someone throughout your life (which is what the marriage promise is usually taken to include), then you have an obligation to do so (unless the person to whom you make the promise releases you from it) even if you and she could find greater happiness elsewhere. Second, there are ethical considerations that privilege oneself without necessarily conducing to one's own happiness -- the right to lead one's own life as one sees fit, for example. If you are drawn towards one person rather than another, that is itself a reason for pursuing a relationship with that person even if you would be just as happy (or even happier) with another. And third, there are considerations that privilege some basic needs of others that don't have much to do with happiness -- the need to retain a certain amount of self-esteem, for example. If you couldn't break with your first girlfriend without destroying her self-esteem, that would be a reason to stay with her even if you were confident that her long-term happiness would benefit from the break.
For all of these reasons, I can't provide a 'yes' or 'no' answer to your question. But I hope I have given you a richer framework for thinking about the situation you were in.