All your answers can be answered through this book “how to stop stuttering and love speaking “ by Lee Lovett. Only $10 for the ebook. And no it does not have to do with the diaphragm. Long answers short it has to do with anxiety. You get anxious when you’re around people, talking on the phone, or even drive thru maybe and that causes you to start thinking words which is one of the big reasons we stutter. We don’t stutter alone because your anxiety is close to zero