Lack of self confidence and probably some anxiety in there, too. I've had trouble with both of them and I think the surest way of fixing it yourself is doing something you're good at with a group. I used to be sorta introverted and not real confident in any of my abilities at all, especially making new friends, talking to girls, etc. But I started weight lifting after school (in high school) and met some new people, I also got a job in sales and came to excel at it and made a ton of friends there, too. Once you find a few cool things that you're good at and you can share the experience with others you'll gain that confidence and break through that wall of social anxiety. You're not crazy or weird or anything, you've just got to break out a little. My suggestion is to get a job if you don't already have one, and join a club at college. Not some computer club, either, something more social like politic, philosophy, something like that. Start lifting weights, great way to meet new people (guys and girls) at the gym. Learn some tips online and share them with people there. Everyone is friendly at the gym! Maybe try and play some ultimate frisbee or something, too. People play it a lot at my college and it can be really fun, even if you're like me and not very good at it.
Actually, I suppose I've got some good experience then from conquering my own issues with self confidence and anxiety. If you want some advice from me feel free to hit me up, I don't mind a bit. You and I were in the same shoes, really. It's all about breaking out, trying some different things, meeting new people. You'd be surprised how if you hang out with some new people for a couple of weeks how quickly they'll invite you to parties and stuff. Great way to get better at socializing and meet new friends.