This very moment is all you have. It is all that exists. The past, the future, do they even really exist at all? I was listening to a podcast just yesterday where they were talking about that kind of stuff and how to re-frame your way of thinking to be happier and more successful.
Basically dwelling on the past is just lost energy; worrying about what could you have done, or what should you have done is a waste because you can not go back in time, and trying to predict or make perfect choices in the future is also a waste because you really don't know what's gonna happen, who you will meet etc.
It's the hardest thing for me to do is just let life ride and not worry so much about the past or future, but I still think it's a good idea of course to try and make smart decisions, live below your means, have healthy relationships with people and ditch the bad influences and negative people etc.