Why can't I get her out of my mind?

You should stop being in love with her. I know it's not easy, it won't, but if after all those years of friendship she doesn't feel the same way you do, then you should try to get over her. That doesn't mean to stop been friends w/ her. Usually meeting and dating a new person will help forget about the other... go out there and try to hook up with someone new. One of the main reasons people are so hung up on certain people and ideas is because they don't have that much of a life, leading to them to constantly think about the same old. Go out more, get involved in new activities, meet new people, try to make new friends, that will definitely help you get over her.