It may sound hokey but in my late teens, my mom has kind of reached the end of her rope and hired a hypnotist. I went once a week and would sit in a chair listening to a relaxation tape. It would help me regulate my breathing, run through muscle tightening and relaxing, and all the while, the hypnotist is mumbling something through something... honestly I don’t think there was any “hypnosis” involved. However, I got a tape of that relaxation cycle and listened to it nightly. It’s been 15 years since I’ve listened to it but I can still recite much of it. Honestly, look for some relaxation channels online on Spotify or something and start getting your breathing under control. I’m not saying what works for one person works for everyone but it can’t hurt to try? After the hypnosis sessions, my stutter went from moderately severe to mild.