In the words of Rene Descartes, "I think therefore I am." It suggests that as long as there is thought, we can be sure that the thought, or the person thinking truly exist. For thought cannot exist if there is no one is thinking about it.

I don't know whether I truly exist in the past, it could be a mere construction of a new memory , instead of reconstruction of a past memory. I don't know whether I exist in the future in a similar fashion. Because what I am thinking now, whether it is the past or the future is just based on representation of my current thought. Everything could just be a dream.

If what Descartes says is in fact true, at least at the very present moment, I know that I exist. Similarly feelings/emotions are presented in the same way too. I don't know what I felt in the past or in the future, but I do know that this feeling is present now.

I could imagine being happy in the future, I could imagine being happy in the past, but I cannot confirm both with certainty as well. But I know I can be sure I am feeling happy at the present moment.

Perhaps ultimately only the present matters.