Yours sounds like more of a mental block type impediment, but I've stuttered my entire life and, for me, it is a genetic thing, cause by miscommunication of the parts of the brain that think words and make speech, most likely. There are a good number of causes but yours is likely sort of anxiety induced which is rather common. Funnily enough I am perfectly fluent when talking to myself, or completely by myself. Hell I can even rap. But the moment I get in front of another person, - wham!- I'm hit with all my blocks. I don't understand it. But it doesn't get in the way too bad anymore. I know this likely wasn't helpful but hope you were a little entertained. :)