I sometimes stutter when I am very nervous. This means that it happens in places such as crowded shops, which makes it all the more embarrassing!
I first developed a stammer when I was about 18, and I became very self-conscious about it very quickly. I can remember the day when I phoned one of my friends and said "H.. h... h... ", and he said "Oh hi Patrick!". I felt absolutely crushed.
Recently it has come back, which I can only attribute to my increasing anxiety. One thing I have found which helps me is that whenever I start to stammer, I simply start take a breath and start the sentence again, taking more care about choosing the right words. It works; and what's more, most people don't even seem to notice that I have done anything strange. When you start to listen, you'll hear that other people start their sentences again, go back on words etc., all the time.
I wish you the very best in coping with this embarrassing problem. You're not alone. Look after yourself.