Why can't I find a girl who wants me?

Hey everybody, I'm 18 years old and a freshman in college. I'm not horrifically ugly, but I'm not a looker either. I've never had a girlfriend before. I've been on a couple dates but I'm a **** up. I always end up screwing something up or I'm "a good friend, but not boyfriend type". Part of the problem is I'm not outwardly shy, but I have a lot of trouble letting people get close to me. Everytime I try to open up to a girl, she just runs because I'm weird or something. The more times I get rejected, the harder it gets. I haven't been on a date in a few months and I feel like giving up. If someone could help me out I would really appreciate it. Thank you.