Is it normal that i'm thinking about someone else other than my girlfriend?

Several months ago I returned to my home country after a several-year stint half way across the world. On my first night out, I hook up with a girl who eventually becomes my girlfriend after two months of me fooling around and having fun (mind you I just got out of a year-long relationship), but before I got together with her, I hooked up with a girl I had a crush on during my middle school years. Problem is, I think of her more than my girlfriend. IIN? How do I forget about her?