I learned a very helpful breathing exercise. It uses abdominal breathing training. It was introduced to me by my mindfulness/self-hypnosis trainer. You inhale deeply from your abdomen either 3 or 4 counts. rest( hold) one count, and then exhale for twice the number of counts as you inhale. If you inhale 3 counts, you exhale 6 counts. Then rest one count and repeat. This has helped many stutterers and also trained professional singers. Good luck.