You've just described me. I get the exact same feeling. When i want to talk to someone i get these strange anxiety attacks they're hard to describe I feel really twitchy and it makes me want to leave the conversation. Nobodys ever called me dumb, but i have been bullied for being so quiet or getting really twitchy. I kind of think that my problem is that I over analyze things in my head...then when i go to say what i want it comes out the wrong way, or I'll mumble or something. This is really holding me back. I have heard that most people that are quiet are quiet because they're unknowledgable, i read this in a book and its been bothering me alot. I don't think im dumb i get good grades, i like to play music alot. I think something that will help is exercise(when i work out i dont feel as twitchy and you feel better about your appearance), also reading alot having a big vocabulary helps,
stay informed watch the news read the paper, try and throw yourself in uncomfortable situations(i know its hard, but i try and do it all the time, ill admit that i fail alot but you'll feel better knowing that you tried.) The other day i asked this girl i liked if she wanted to hang out... and she said yes... i was so happy, but then when we hung out it was a little awkward...chemistry over msn messenger is no good( you should get rid of that program and use the phone). Thats the best I can think of
Goodluck to you I know how it feels I've felt unimaginably awkward many times and full of anxiety that i feel chained. I'm a prisoner of my own mind.