I do this alot myself. I think up a bunch of different scenarios and play out how I will react to them. Like you mentioned, I find it hard to react in a positive way or any way at all to the 'little' things that happen to make others react. I'll often just stand or sit there, confused and left out. I pretend alot when I'm out with people, I usually just follow along to what the people around me to doing. It makes me feel really silly most of the time, but I think if I just keep pretending, maybe one day it wont feel forced, and it will be natural. Sorry, this probably isn't the best advice. Just wanting you to know you are not alone