Remember the cliche about forgetting so you can move on? I realized that we shouldn't really forget our mistakes because if we do, we might end up doing it again. Remembering our failures and shortcomings, I believe, would help us move on and be better. Doing so will keep us on our toes. Sometimes we need a constant reminder on why we are trying to do better, why we are trying to make ourselves better. So when you find yourself remembering a negative past, maybe it's your mind's way of saying "Hey, remember when you did this?". Pause for a while and reflect on where you are now. Maybe in trying to forget the bad, you also forgot why you're trying to do good.
