A tight diaphragm is a symptom of how you feel, not the reason why you feel this way. I've got fantastic abbs due to stuttering for 55 of my 75 years. My best advice is: find a way to forget yourself while addressing a person. What do you notice that is special about this person? What is interesting about her/him? Believe you really care about this person. Concentrate all of your attention on making as real a connection as possible. Think KINDNESS. My best result came when I reached out and touched the guy on the arm. All my tension and fear instantly evaporated. Remember that you are equal with everyone around you. You are not unique in your fears and worries. Everyone likes talking about themself, so ask questions; then really listen without planning what you're going to say next. Enjoy the feeling of being connected to another human being. Stuttering is a phobia, an irrational fear; not a disease. I know this because I was there for most of my life.