As an on-again, off-again stutterer, I know this. Every time I open my mouth, I can potentially face disaster. Grim way to live, if you think of it in those terms. I don't, because to do so would end up with my living as a hermit in Siberia somewhere.

"Performance anxiety" has extensive shelves of long-winded tomes to allegedly help you through. Taken as a whole, none have done a damned thing for me.

A lifetime of this torture has gifted me with one solution: Ignore the feelings. Just don't go there. If my head insists on going there, I immediately do a few things.

Take the pressure off. I'll admit to my audience that I have a speech impediment, and to extend me the courtesy of a little patience. Once I see everyone relax, my issue resolves almost immediately. My fear is not my own disfluency, but others' judgments.
Pinch the crap out of my leg or side. Redirect my brain from 'freak out' to 'damn, that hurts'. Works every time. If my brain wants to go back again, I contemplate a stamp on my own toes ... but just the threat of that pain prevents further misbehavior on the part of my tongue.
Ask myself if I've misinterpreted the feeling, and is it actually excitement or anticipation. In our modern anxious world, we as adults are actively forgetting these feelings. And many psychiatrists don't want you to remember them (for sheer drug profits). IMHO.

After a crisis, I immediately do three things: 1) FORGIVE MYSELF, and remember people like me for who I am, no matter the disfluency. Everyone has a bad day; this was mine, and that's OK. 2) Reduce caffeine/alcohol consumption, and 3) increase physical activity. I find that for me, disfluency is strongly linked to self-confidence, and my self-confidence is linked to my state of fitness. Caffeine generally enlivens nerve/anxiety-channel 'energy' levels, sabotaging fluency similar to having the Marx Brothers go coconuts on the basic functions of tongue/mouth/throat. Alcohol - not the night of a drink, but the day after - can have exacerbating effects. Before important speaking requirements, I go completely without. If disfluency/anxiety persists, I won't touch alcohol for months.

Just my $0.02.

Later: There is one other circumstance I forgot to mention. My brain will begin to sabotage my fluency when I'm unchallenged or bored. If I am performing at a high level, and have been for a while, my brain decides, without my consent, to destroy fluency and force me to change circumstances in order to renew the challenge and effort required to perform to a creditable standard. This was always a problem in school for me – I'd read the course syllabus(es), the required books, and be bored to death for the next six months waiting for the rest of the class to just do the damn reading so we could learn something else.

Even later: My other secret weapon. Performance anxiety hates ridicule. If you can laugh at it, if you can find the humor in the situation, you gain power. Rafael Sabatini, Scaramouche, Book II, Chapter V. If you can 'work it', you can beat it. With a stick.