I am always becoming obsessed with girls.?

I am a Junior in high school and I have a problem. I have not had the best luck with girls. When I find a girl that is really cool then it seems like I become obsessed withh her. I start having all these fantasies of us being together (not sexual. I am constantly thinking of her. Whenever I see her with another guy I get majorly jealous and feel bad. I just dont understand how not to become osessed with this girl. I lack self confidentenI just dont know what to do.ce, and often I think she is better than me and why would she want to be with me and I feel depressed for periods of time.