Meditation. Lots of meditation. You have to control your breathing. Try box breathing or imagine your breathing in a wave like a dolphin rising and falling. Play some lo-fi or white noise(or green, brown, whatever) while you are in bed for the night and focus only on your breathing. If your mind wanders, pull it back to your breathing. You are always going to stutter and it’s at different severities for different people, but… controlling your breathing and relaxing your mind can do nothing but help. It helped me quite a bit. I still stutter, especially when I’m stressed, but it’s manageable.