18 and never had a real relationship...what's wrong?

I'm 18 years old and have only had one boyfriend (and that was back in middle school which no one even counts anymore) and I've only been kissed once (also back in middle school.) I've only been asked out once since then but said no. And Idk why but things just never work out with the me and guys. Either I don't like them back(though it's only been a few guys who I've known liked me) or they don't like me "like that". I'm tall (5'11) and I don't think I'm ugly. I mean I'm not like stunning or anything. I've been told that I'm pretty and could model...I don't see it though. Admittedly, I am really shy and I guess on the "smart" side. Could that be the problem? What's wrong with me?