What can i do ?? I cant stop thinking about her?

Don't tell her exactly how you feel, just hint it. For instance, when you are alone (preferably in a romantic moment), you could say,"You know... you're boyfriend... he seems like a nice guy..." and sort of look a bit disappointed but try to act like your hiding it, like you are nervous. Then say "If I had a girl like you though, I would give her flowers every day. I would tell her exactly how I feel, how much I love her" and tell her things like that. Then, stand up, pause, and then look at her and say "I mean, that's what I would do..." and hint her. Girls love hints. But do it sweetly.