I think a lot of “us” have same experience as you. The severity of my stuttering is highly depend on my environment and my mental stability, there are times where I don’t stutter at all because I’m in a good state, but as soon as things turn south in my life my stuttering gets worse. There is no sufficient explanation for this as stuttering couldn’t sufficiently get connected to social anxiety. I taught myself some comping mechanisms such as the 47-11 method (a breathing technique) to calm myself down before I have to talk in front of audience, but besides that I can no other advice than trying therapy.