It is pretty hard to forget about the past but it is one of the best things you can do for you and your mental health. It is one of the steps to your healing process because the reality is some of the things that hurt you, if you do not forget it would forever haunt you. For your sanity, you just have to forget everything and move on.
There are other situations where people tend to forget. You might be giving your best to a particular person and the moment you fail to meet up to their expectations they fail to remember the times when you have always been good to them and it is sad. You just have to do your best and do not feel bad whenever you say No. You cannot always please humans and that is okay.
At the end of the day, do not forget to appreciate people but remember to forget the past. Focus on the present, ignore the past.