I used to spend an inordinate amount of time worried about past mistakes, or people who were unkind to me in the past. Then I realized how absolutely unproductive that can be.
A lot of people keep talking about how important it is to "live in the now", to always have one's head in the present instead of the past or future, but frankly, I find that difficult. These days, I'm usually ten steps ahead of myself, plotting strategies for future actions. And I don't see anything particularly wrong with that. I have a friend who keeps raving on about "living for today", but he hasn't got the slightest clue about what he's going to be doing for his retirement (which is coming right up). Living in the "now", for some people, is like an ostrich putting its head in the sand...they don't think about the future, because it's just too scary. But any sane, rational adult MUST think about the future.
Having said that, don't get too consumed with the future, because fate is unpredictable and can throw out obstacles you never expected.
To utterly forget about the past is also an act of escapism. One doesn't have to dwell in the past (which is definitely unhealthy), but the past cannot be forgotten, either.
So my answer is to get on good terms with all three--past, present and future. They're all a part of your life, and must be coped with and addressed on their own unique terms. Each has its own lesson to teach, if we're wise enough to be good students.