Thinking about the past is one way of coming to terms with where you are now and how the choices you made and also the choices you tried to avoid to make helped you get where you are today. Besides the random unfortunate circumstances we sometimes encounter, it is pretty much our choices in life that put us in the place we are today.
Holding on to regret, sadness, anger or any negitive emotion from past choices only hinders one from making better choices in the present. So feel those emotions, own them and learn to forgive yourself and others so you can move forward. Every morning you awake is a clean slate if you let it be and a new chance to make your life better and also another chance to be appositive influence on the world you live in.
For me the hardest thing to do is forgive myself of my poor choices and mistakes I had made in the past. I learned that by hanging on to such negitive thoughts and emotions I only created a negitive self image and a somewhat defeatist mentality. So now I just try to own my mistakes, face I made them and accept what is done is in the past. I also have to remind myself that I am not perfect and that I should never expect myself to be such. I Remeber the mistake, learn from it and try to forgive myself since I am just human.
Though we can not undo what we have done wrong in the past, we can how ever try to make up for those mistakes in many ways. You can try being of service to those in need and by being compassionate and caring to all. If you are able to you should also make amends to those you have done wrong in what ever way that is possible. In the end you will have to forgive yourself and also forgive others in order to free yourself up to live your life to the fullest.