You can, it just feels like you can't Nobody is holding your mouth shut. Everything is something you are doing, that you have learned to unconsciously do, probably when you were a kid. The good news is you can unlearn it and this is where a very good speech therapist comes in. This isn't something you can work on though in difficult situations. You need to find the easiest situation where you might block and work on it there. The bottom line is that you block because you don't want to stutter and the only way out of that is to let yourself show stuttering.