OP, I can totally understand you. I feel exactly this way, I avoid going out with people if I can. I never know what to say so most of the time I just remain silent. The only people who I'm confident to talk to are my close family and my best friend.
I have recently being diagnosed with Asperger so that's the reason for me. I don't feel strange or awkward, because in my world the awkward people are the ones partying and doing such other social things. So I feel happy if I can remain at home doing my stuff.