I agree with mecanix. The past is done, over with, finished. The only thing that you CAN do is focus on today, this minute, and making good decisions now. If you try your best to make good decisions in the here and now then your future will be bright. I struggle with this myself. I find my mind wandering to thoughts of how much time I've wasted. Then I remind myself that I learned many lessons along the way, many things that no amount of money can teach you. I learned to be grateful for what I have, which is A LOT considering how much damage I did to others, and myself over the years. I hope that you can think of some things that you are grateful for when you start to feel bad about the past.
God bless!