socially inept?

so, today i went out, with muy little sister to a friends birthday gathering.
it was nice, i mean, everyone was chatty and talkative, but i cant help feeling like i dont fit in.
i feel so uncomfortable being out and talking.
i talk because im scared of looking nerves/like a looser, yet i come away feeling so alone that it was a pointless effort.
in my head im wondering if they thought i was stupid, an idiot, horrible, weird... im wondering if i looked like a greedy pig eating a bit of birthday cake, or if i looked fat. im wondering if i should ever try and socialise again, because it feels like i fail miserably at it.
i wish i felt more comfortable being around people, but it feels like torture. i hate being alone, and feeling by myself, but im so petrafied of peoples reactions that it feels more comfotable feeling sad and alone, then risking being upset.
im so scared these situations will turn into school ones, even tho i know thats ridiculas. when im in crowds, i panic because at school crowds only meant being beaten up or verbally tortured. when i have to introduce myself, im petrafied that people will dislike me for no apparent reason, just because im me... i dont know how to unthink these things, and im scared to try incase its all true anyway.
tonight im feeling pretty low. my sisters asleep in the next room, and im wishing i was alone so i could purge/cut/cry as much as i needed. im wishing i wasnt like this at all. wish i just wasnt here =-(