Sincerity

What is it that makes others view us as insincere?
I have been told that I have a very vibrant and extroverted personality. This has been a blessing and a curse. The blessing of it, I would say, is the fact that I have very little fear of approaching others and getting to know them. I love to be in groups and I enjoy the spotlight of social situations. The curse of the matter is that I am often thought of as being fake. That i have a mask or a facade that I put on in certain groups or situations.
Another fault of mine is the fact that too often I care about what others think of me. I, in my attempts to have honor and respect, that my predecessors found so important, always seem to care when the world thinks ill of me.
Although one might attribute the thoughts of one being fake as a personal judgement that is fickle and need not be given attention, I find that implied insincerity can often hurt a friendship's trust. When a friend gives a complement, the trust isn't there and so it's just words. Trust is the integral factor in any relationship, I dare say. So what does one do to regain trust? How can one convince others that he is, in fact, sincere and honest?
It seems that, often, when one loses his good name, it is lost forever. I only hope this is not the case with me. If so, I imagine new friends and circles would be needed.
Perhaps, I can make a habbit of undersigning all of my comments with "Seriously" or "I mean it." This may be a little extreme, but I dont know that any effort will be useless.