If you are sad be sad, if you are angry be angry, if you could care less, that's okay too.
Whatever you feel right now don't try to change it. Go somewhere quiet, sit down, and allow whatever you feel be how you feel. If you start to think of other things- even for just a minute than notice that that's the nature of your mind…and that's what's eventually going to happen completely: you will think of other things, because there is more to life than just sadness, anger, and one girl.
But don't try and force yourself to forget or feel bad, cause that'll make it worse (if not immediately than in the long run).
You got attached to a person and a situation, welcome to the human race…you'll eventually lose everyone you love, so take this as an opportunity to be cool with that.