History is an illusion at best. At worst, it is a means to justify more of the same. — Zumwalt
Perhaps thinking of reality divided up into what was, what is and what will be is nothing more than a convenient interpretation.
If we admit of the possibility of existence, at the highest level, being outside the realm of time, space and energy, then its plain silly for us to reflect on how the past could have been different.
If we are but a cause-and-effect organism (or mechanism), and we can move only forward along time, then it is even sillier.
I believe existence is existence and there is no existence then, existence now and existence later — there is just existence.
Don’t worry about the future. Prepare, when necessary, but don’t prepare for the future. Just prepare.
Don’t try to fight memories or shrink from past events. The most harm comes from running away and resisting. Accept and celebrate.
Don’t even worry about “staying in the present” or “enjoying the moment.” Just be there. It is possible “there” is past, present and future.