Shambhoo, people who grumble about your accent don't matter to you. Those who matter will politely ask you to repeat if they don't understand. You are doing no harm to yourself when you don't speak English in American accent, as American tongue is shit anyways. Try to pronounce each words separately and people will understand everything. I can guarantee that your English is much better than most non-Americans living there. Perhaps even better than some Americans who never made to the school.

As far as being nervous at Social situations is concerned, I'd not give a flying f*** about what others might think about me. And trust me those people have their own problems. No one would bother about how you were doing, unless you are there for a job interview or fixing a girl for yourself. Whenever you go to a gathering, just loosen up yourself. Talk to someone if you want to, otherwise enjoy the atmosphere by smiling at people and moving around. But don't think what you'd talk with x,y,z. Nobody talks serious in such gatherings anyway.

I'd also suggest you to watch Ali G clips in youtube and try to impersonate him in front of your friends.