Work with a speech therapist if you can. That may be the most helpful thing. Secondarily, you should try to figure out what are the triggering conditions - certain syllables, certain scenarios (public vs private speech for example), certain body posture or mood. If you pay attention, you will realize what these triggering conditions are and learn to avoid them. Eventually, you will come up with tricks to combat them. I am in my 40s and it still happens occasionally. So be patient and be mentally prepared to fight it for a long time. It does get better though. I work in PR and talk for a living, believe it or not. Don’t let it define you and limit your dreams.