Hello CaptainCab, what you have described is something that we all go through - putting on different masks for different people to suit the occasion. This makes sense sometimes, you talk about being serious and respectful when going into groups of people you don't know: this is the cautious and sensible approach. Where it sounds like you're struggling is that you are second-guessing yourself a lot and worrying what others think of you. Here's a very important true fact that might help you relax a bit when you are out: EVERYONE else is thinking the same thing. But we are all experts at hiding it in case we get found out. Chances are, unless you do something particularly memorable like tip a pavlova all over the front of someone that the big social gaff you think you've made will be completely unnoticed by everyone else.
We all 'warm up' more as we get to know people better, this is natural and there's no need for you to force a 'party face' if it doesn't feel right for you. Some people do this naturally, but often this is a mask as well - I have known friends who always seem to be the life of the party, but in secret will tell you they are terrified at social occasions.
If I were you, I would keep going the way you are, but don't make assumptions about what others think of you (the comments you make about people not getting your humour or giving you the cold shoulder for instance). Ask questions when you meet new people, peopel always love to talk abouit themselves, and keep going until you happen upon a topic you have in common. Then bingo, you are away. If you're not hitting it off, move on as politely as you can.
