yeh i'm sort of in the same boat as you, i can show up to a party but it's the trying to talk for hours and hours like everyone else that feels like climbing mount everest, and so then i get this reputation of being boring, and i know my firneds are that judgemental enough to say it behind my back, for instance when girls have been interested in me ive had comments from friends like "i dont what she sees in you" ..."hey guess what she has a crush on you but she's a book worm, you wouldnt be stimulating enough to last long, you'd be eaten alive by her wit" ...."your average looking, i dont know why she would be interested in you" ...etc etc.
i'm not a big talker unless i'm full confidence which is rare because looking into the past i don;t see "ME" as a confident person, but a person who runs from everything, even around family i don't say alot or feel confident expressing myself.
but yeh, parties, pubs are an intimidating environment, you have a mixture of sincere people wanting to enjoy their free time and those who want to meet new people and hook up, some people are blindingly good looking, so you compare yourself to them and then there are the strong characters who everyone wants to be around, you compare yourself to them and wonder what it must be like being a person magnet.
it's just very difficult if you don;t have at least some "Gift" for socialising, whether it's looks, character, personality, and i feel that i lack in all those departments, and i think reactions from friends over the years confirms my beliefs.
it sucks, but i do try, i just need to stop thinking about what others think so much, that's easier said then done though, good luck!
what ive noticed is that people seem to have so many life stories and experiances and their memory of movies, jokes, i find it astonishing how some people can just "talk, talk, talk" ... so effotlessly, i envy them