I've been having problems lately about letting go of the past myself. It can be hard sometimes, whether it is about a girl or about your life in general. The best thing you can do is let it go, and move on, but I'd be lying to you if I said I could do that myself. The past is something that should be cherished for what it
was, and not forgotten. I mean sure it can make you sad, but can you change that? Sometimes that answer is yes, most of the time it is no, which is why it makes you sad.
How I've been coping with my sadness is I make a goal for myself. Think of something else that would make you happy. Like say "Alright so that girl used to be great, and now she's not; well even if that didn't go so well, I am going to a new school which is a new opportunity for me to meet new girls, some of which may be even better than her." Moving on isn't easy, but it's a part of life. We can't let our past chain us down forever, or we'll never be happy.