When I started doing stand up, and then now when I started to present myself in cosplay presentations and contests on anime conventions, I always thought I would mess up, I would do terrible on stage, I would forget the whole thing, and that fear dominated me until I got there. When I climb the stage, I am in control of my fear. Things are not that bad. And you know what? I've bombed. I've messed up some times. I had problems during a 30 minute cosplay presentation in which the audio started to jump and the whole thing was set to a big disaster. And all those times, you know what's the worse that happened? Nothing. Nothing bad happened. I'm still alive. People watching couldn't care less. Even if things go wrong, it's okay.
That's the same for social situations. First, you have to dominate your fear by facing it. Seeing how it's not that bad. And being yourself - that dosen't mean you're "faking it" or being someone you aren't, but you're restraining yourself from the things you want to say, you want to do. I bet you refrain from mentioning stuff you consider geeky or nerd-like - well, those are part of what you are (and as a matter of fact, of what we ALL here are), and you should be able to talk about it with no fear. Be yourself, really. Don't be afraid.
And, also, what is the worse thing that can happen? Think about it for a while: if some girl you are not interested comes to you and says she wants to go out with you on a date, you won't ridicule her. You won't do anything cruel. You'll try to reject her request the most politely way possible, and even feel a bit special for being someone that some girl wanted. You'll probably befriend her. So, why would someone behave in a different way? Why do we always judge that people are meaner than we are, tha people will hurt us in the most horrible ways? If someone has hurt you before, it dosen't mean everyone is like that. And that person probably didn't do it on porpouse, it was something that happened in a moment in which you took it in a bad way, in a way that hurted you.
So, there. Seek profesional help. Let your parents know you're doing this, they'll help you the best way they can. Be yourself, and I mean, BE yourself. Be the same person you are when you're alone. Don't censor your thoughts for fear of being misunderstood or ridiculed. And relax, remember, nothing terrible will happen if you try to be more social.