you need to practice breathwork, diaphragmatic breathing is particularly good. A lot of the stutter issues I had came form the way I breath and the more I worked on it, better it became. Find some videos on youtube, you can even practice it lying in bed when you wake up or before you go to bed. It's also good to practice it while standing up or sitting so that your body learns how to breath better in all positions. It's a slow process but it's worth it! Doing yoga is also great with proper breathwork.