I don't know why I keep thinking of my ex grifriend?

When i always go to sleep I always think of ex girfriend I don't know why? And I still love her as much as i used to love her when we first went out with each other. Now since she with willie I suppost to go out with another girl. BUT THE PROBLEM IS I keep thinking of her not the other girl. So if anyone know PLEASE ANSWER THIS QUESTION.