You have to push yourself slowly to be more of an extrovert. Try to talk more to people you don't know, about things you usually wouldn't talk about. People have probably tried to reach out to you, but judging from your post you have probably indirectly rejected their advances. After a while people will either A. think you don't like them or B. simply lose interest after being rejected by you enough times. Next time someone offers you a chance to become closer to them or an invitation of some kind, suck up your pride (or whatever it is) and accept it. Even if you are extremely uncomfortable or have a really sh*tty time. At least that way you show that you are at least willing to try to get to know them better.
I just graduated from HS and I had to do this after moving to a new school my sophomore year. I was never great at meeting new people, so when I started completely fresh it was very hard on me at first. I had no friends for about 6 months because people just assumed that because I was outgoing that I had friends. I had to push my social limits and basically force my way into a new group of friends. I found someone that I found interesting and met up with them at lunch and continued to do so every day and after a while this person grew to know me along with this persons friends and pretty soon I found acceptance within a group of people. I know share an apartment with this person who I consider my best friend. It can be very hard and stressful at first, but just keep your head up and keep working hard at it.
Sorry in advance, Earth616, this isn't a complaint against you or an intentional diminishment of your difficulties. If it offends or insults you, I sincerely apologize. Now that said.
WHAT IS WITH ALL THE SUPPORT GROUP STYLE THREADS LATELY?!
Seriously, it's like one every other day now. At least. Isn't there a thread or something around here for all personal problems such as this to be posted in?! It'd certainly save us on InnerAssassin threads. Maybe.
M'kay, resigned complaint over. Back to the problem with Earth.
Sounds like you DO fit in. You sound relatively well adjusted, generally well liked. You even openly state that you're asked to go to parties, though you decline the invitation. You certainly don't sound like you have problems with self esteem, or general appearance. Attitude definitely isn't a problem...
Aside from the apparent nausea at social interaction, you should be fine. Yet you're not. The problem apparently lies in the environment, and your regard for it: In an environment where you absolutely HAVE to interact with other people, you have no difficulty doing so. Work, school, etc. However, beyond those areas, areas where interaction is optional, you become physically ill at the mere idea. So what's the solution to the problem...
Try speaking to someone in your school environment. Get to know them there, in the school. Become friends with them where you don't have a problem. Then, eventually, they should ask you to interact outside of school. Force yourself to allow it, despite feelings to the contrary. Once the appointment is made, and you can't go back on it, you are forced into a situation similar to school and work: In order to function, you HAVE to interact. So, just like school and work, your natural charisma and attitude should kick in and you'll be fine. Wash, rinse, repeat.