Breathing is so important for stuttering and keeping calm. Take note of when fluent speakers take a breath in sentences. I think it’s the pressure to get the words out that we forget to breathe. I didn’t realise until quite recent, I’m 41, that you’re meant to breathe through your nose. I found the book Breath by James Nestor really insightful. Also exercise, running and weights can help you to focus on your breathing, it really helps to be more aware of your breathing patterns.