Breathing WAS a help for me, but it came through meditation. I literally listened to the same cassette tape (I’m old) for years, every night. It was a meditation tape that focused on breathing patterns and the tightening and releasing of muscle groups all over my body. I haven’t had that tape in probably 15 years but I can still hear it in my head. It was a breakthrough for me. Obviously it’s different for everyone but learning how to relax my mind and body allowed me to find a path toward managing my stutter. Like a smoke I could blow on the hive of bees in my brain to calm it. I’d contend that even if breathing exercise and meditation don’t help your stutter, it’s valuable anyway. Good luck!