I left another comment but deleted it due to me kinda rambling lol. But I started doing BJJ and grappling when I was 16-19 and it helped me personally a ton. More than speech therapy did. I think it was a combination of having an activity I was good at and could do, it was physical so it was very tiring and it made it where I seemed to carry less tension around with me all the time, less clenched up if that makes sense. All that cummulated to increase my confidence overall. I'm 21 now and I had to quit due to college, and I notice I stutter 3-4 times more than when I was grappling. I think confidence overall leads to less stuttering. And I noticed when I was grappling I didn't even really experience that fear you get before speaking to someone new