Damn i also feel this all the time. The only thing that works for me so far is to speak as slowly and relaxed as possible, but even then i get constant blocks all the time, even when i'm alone. Only diffrerence to you is that i actually think about what to say fast and i already know what i will say, i have it on my mind but when the time comes to actually say it out loud, i just can't without stuttering. It has been getting worse over the course of this year for some reason. This is so fucking stressing.