Trust me, you're talking to the worlds worst at that. I don't really know how to play chess but i'll still make this analogy...in most situations, even just ordinary everyday ones, i'll analayze something and think about the outcome for each move or step i could take...just like you would in a chess match. Does that mean I still make the right choice? hell naw, lol.
If or when someone comes up to me out of the blue it DOES throws me off sometimes, it means I have to just wing the situation because I haven't planned it at all...I normally know what i'm going to say before i say it and have already thought about what responses i could possibly get for each thing I say, haha :-p Do i think too much?

The more that happens the better you'll be at it, as everything it takes practice. Is there a way to practice quick wit? I always say it's not the situations you get yourself into but how you handle them. Get comfortable interacting with strangers that you've gone up to and you'll be loaded for those that come to you, no running off with your tail between your legs. Now go watch 40 year old virgin, the library scene is priceless.
It is ok to be reserved, there's plenty of others out there the same way...you'll change that if you want or feel the need to but opening yourself up too much or too quickly can have negative effects too. Gotta find that happy medium you're comfortable with.