Hmmm, this is pretty complex, Vixen. I used to have the same problem, but things radically changed around my second year at college. I think it all changed around the time my self-confidence issues stopped. As soon as I started feeling completely comfortable about myself; I noticed I paid less attention to guys, I stopped over-analyzing everything, and things with guys became so much easier; like it all started flowing naturally. It sort of scared me in the beginning, but it's funny. I guess people do pick up on your emotional state; even things you're not necessarily aware of yourself.
But well, of course, everybody's different. I guess there could be a lot of reasons to this. This was just my personal experience