For me, it’s definitely a confidence issue, perfectionism, caring what others think, etc. It’s 99% mental and 1% breathing. When I was in my early 20s, I was making good money, partying, almost living a rockstar lifestyle. I don’t remember stuttering or at least I didn’t care about it. I was very confident in myself, almost to the point of being a dickhead. Now I’m a lot less confident and my stutter is a bit worse than it’s ever been. If you can speak fluently by yourself but not with others, then it has to be psychosomatic.