Absolutely completely have this problem. I've been diagnosed almost a year, and am only just starting to be more open about it. I was fairly convinced for a few years before diagnosis, but the few I mentioned it to were very skeptical, and in one case actually laughed at me for suggesting it. Being a girl, my social mimicry is better, and over the years I've developed lots of ways to fake being normal, but I'm SO different at home. I can walk my dog with a friend for an hour, and appear 100% NT for that period, but that kinda makes it worse. It's nice that I can feel somewhat "normal" with them, but then it kinda prevents them understanding just how disabled I am by it.
An awful lot of people from my past just haven't been told, because I've already had the reaction of "nah, you're too normal to be autistic" and because I don't have the emotional strength to defend myself to people who don't know much about the condition who are still sure I can't have it. It's especially hard because I'm BRILLIANT at written communication, so if I'm talking to someone through facebook or whatever, I sound completely normal to them, but then always mess it up face to face :/