There are plenty of philosophers who have not agreed with Descartes' line of thought here, though they are not "ancient" philosophers, as Descartes did not propound the "proof", if that is what it is, until 1637, in the Discourse on the Method and, in a slightly different form, in 1641 in the Meditations. You can find some interesting material in the "Objections" to the Meditations , for example the Fifth, by Gassendi, or the Fourth, by Arnauld. Hobbes too, in the Second Objection, makes of Descartes' argument a triviality. How (he asks) can I know that I am thinking? 'It can only be from our ability to conceive an act without its subject. We cannot conceive of jumping without a jumper . . .'. Through the centuries Descartes' dictum has come under even more fire from different directions, for example from A.J. Ayer in Chapter 2 of Language, Truth and Logic. Descartes was only entitled to say that 'There is a thought now,' not 'I think', i.e. 'There is an I and it thinks', because this proposition would make his conclusion a tautology; or it does not follow from 'There is a thought now.' Lichtenberg writes in The Waste Books, K18, that 'We should say it thinks, just as we say it lightens. To say cogito is already to say too much as soon as we translate it I think. To assume, to postulate the I is a practical requirement.' I also think you should take care to separate the questions whether Descartes proof is any good from the question whether the existence of the self is certain or can be doubted. So the phrase "otherwise stated" in your last sentence is not right. The two questions are distinct. Similarly, it is not the question whether the "cogito ergo sum" is indubitable, but whether the existence of the self is.
Descartes sought certainty because he thought that if we know something with certainty, then it must be true. And he was right, if only because 'S knows that p' implies p, so that in 'We know with certainty that . . .' the phrase "with certainty" is redundant; there is no such thing as uncertain knowledge. I suspect that the sense of your question may be Cartesian: is it the case that certainty implies truth? There are several concepts to sort out here: 'We know for sure, or for certain, or with certainty that . . .', 'I am certain (sure) that . . .', 'I feel certain, sure, that . . .', 'It is certain that . . .' (but not 'It is sure that . . .') There is a very useful paper by G.E. Moore called "Certainty" that might be helpful here, which is sensitive to distinctions of this kind. Sean is right in his response above that psychological certainty or "feeling certain" may not be a mark of truth, though I wonder whether anyone has troubled to test the correlation empirically in humans, and whether it makes sense to think about testing it in animals. On the other hand it also seems correct that if something is indeed certain, e.g. that 7×9 = 63, then '7×9 = 63' is true.
Our knowledge of the past derives from perception, memory and inference, in the sense that these are answers to the question, 'How or by what means do you know?' (There are other ways, for example report or testimony). But our knowledge of the future has in it no elements of memory or perception. So as one might therefore expect it is harder to come by knowledge of the future, and we have less of it per hour, if you want. We typically can know more about a past hour than about a future hour, though by no means all of the past hours, for example those in past centuries. If I know p, and p is a proposition about the future, I cannot know it by memory, special cases apart. (A special case would be that I come to know that I am going to Africa next summer - a piece of knowledge about the future - by remembering that I am going to Africa next summer. 'How do you know?' 'I just remembered it . . .' makes sense as a conversation.)
It seems to me, in spite of the assumption you make, however, that in some cases there may not be an element of uncertainty in either knowledge of the past or the future. There is no uncertainty that the cat will be roughly where it is on the sofa in one attosecond - cats don't move that fast - and there is no uncertainty that the cat has been sitting there for the last five minutes, as I have been watching it for the whole time. There is an interesting mistake (I myself think it's a mistake, anyway) to be avoided in this area. Why are there asymmetries in time with respect to knowledge? I am not sure the question put just like that makes sense. Why can we remember the past but not the future, for example? The simple answer is that if I remember something, then it must already have happened, so memory of the future is a contradiction. My own view is that even the alleged logical asymmetries between past and future are much more slippery than they seem at first glance, and we must be careful to get our tenses right. It is certainly true, for example, that the past exists, in the sense that past events have occurred - and what other sense are we considering? But then so does the future exist, in just the same sense: future events will occur.