Feeling pain is no more authentic than thinking that you're feeling pain. It is just that the two ways of accessing the experience of pain are different. When we feel something - pain, joy - we may be not aware that we are feeling it, whereas thinking that you're feeling pain or joy is a conscious experience in which our conceptual apparatus is mobilized. But this doesn't mean that feeling is a more authentic experience than thinking you're feeling. There are experiments that show that injured people feel pain in their amputated limbs even if they know that they cannot feel it anymore. That is to say that you may be deceived by your feelings as well as by your thoughts about your feelings.
As for discerning how we feel as opposed to how we think we feel, I would say that a sensorial experience is always underdetermined. We feel the gap between the raw experience and its conceptualisation according to our previous experiences, our cultural background and what we know about the world and ourselves. Feeling love may be described by a Shakespeare's sonnet or by a simple "Wow!". It is very difficult to disentangle what belongs to the "authentic" experience and what belongs to our way of making sense of it.