So basically me and this friend that I have known for 4 years almost began liking the same girl. We founded out that she also likes the both of us. I tried to forget her and just let my friend have her, but then she keeps on popping inside my head. What should I do ?

You seem kind of young, so you don't know, that if you focus on other things she will slowly fade from your memory. It takes a little time though.