I speak slowly but my brain still moves frantically fast involuntarily, I assume in a similar fashion to the way op describes their involuntary fast speech. I use my fast stream of consciousness to rapidly search for filler words or alternative phrases that will communicate my ideas effectively while avoiding incoming speech blocks (stammers). I don't know if this advice helps but try to channel your fast speech into your internal dialog rather than your physical mouth movements and speak slowly by only selecting a portion of your rapid internal dialog to communicate, hopefully with as few stammers as possible. Best of luck! Try whatever works! Also find a word you know you can say and put it in front of important words that must be communicated that you would for sure stammer on. It's a crutch for sure, but if I put the word, "freaking" (substitute for "fucking" in polite company) in front of any phrase I can pretty much get it out. Obviously not an ideal word to use in a professional setting but sometimes I'll trade looking less professionally articulate to stuttering profusely over a particularly difficult word or phrase.