If I had a time machine I'd go back and make sure Hitler never rose to power, but then would the depression have stopped?
Then of course, I don't know I would have ever been born.
That's why it's folly to think about changing the past. You don't know what could have happened, only guess. And as time has proven countless times, there will be unexpected variables that could arise and take your life an entirely different direction.
Instead of thinking about what could have been, think about what was, and more importantly, what IS. Use the past to explain why things are the way they are, but also accept that that can't be changed. Then, use that past to grow on it. If the past was painful, then see how far you've become and how much you have grown, or will grow. There were ups and downs, but don't let the downs cause you guilt and don't let the ups cause longing. Just learn from it, and continue striving for betterment, because things can always get better.
Also remember that the place where you are now is the product of everything you have been through. It was not time wasted, it's just the time that you needed to become the person who you will be. Cherish that, because if you don't, then no one else will.