Social Phobia

Hello I'm 17 yrs old almost 18 and I suffer from very bad social phobia. I try talking to other people but when I start to say something, I get really nervous. I feel as if I said something dumb or something when that happens. I worry about what they are going to think of me and a lot of other things. What is wrong with me? I think that one of the problems is that I was bullied through-out school, and I still have the worries that the people are going to make fun of me and laugh behind my back. Whenever possible I try to avoid social situations, but lately I've been trying to face them. It's really hard though. I am taking a public speaking course in college now in hopes that it will help me get over my fear of social situations. Does anyone else have the same problem? Please, I really want to get over this problem, if anyone has any advice and/or tips on how to conquer this fear it would be greatly appreciated. Thanks!

P.S. I find it easier to communicate through writing rather than speaking. Although at times when I have to email someone, such as my College Instructor, I get worried about what they will think about my email. This feeling is almost like the feeling that I get when talking. What may be causing this, and is there any tips/cure for it?