One of the most life-changing gifts you can give yourself is forgiveness. Sure, You've made some big mistakes in the past, and did things you aren't proud of. Believe me, most people can relate to this. No one gets far in life without having screwed up along the way.
When looking in the rear-view mirror, the questions to ask yourself are: What did you learn from the painful experiences? How have you grown since then? What are you doing differently now?
If you can look in the mirror, and know that you have grown and changed then it's also time to look in the mirror and say "I forgive you." You aren't your past mistakes. You are the person who is wiser and more empathetic for having gone though them. You are the person who is choosing to rise above your poor choices and do something good with the lessons.
Forgiveness frees you from carrying the kind of negative emotions that keep you from moving forward with the confidence and enthusiasm you need to be happy. When you forgive yourself, you are free to believe that you deserve joy and peace. When you forgive yourself, you put the past where it belongs; you accept your mistakes as only being a small part of your story and you begin chasing the fulfilling, joyful life you were created for.