What are you doing now to build a better life for yourself and contribute to other people's happiness and well-being?
I kinda feel like this problem has a quantitative solution: The more you're doing to put the past behind you and take better care of yourself and others, the less room in your head and time on your hands there will be for regrets or negative memories. You work to replace all of the old/bad with the good/new.
Starting . . . now! :-)