My stutter gets worse with the feedback loop of how I perceive my speech at the moment. If I have to speak loudly, I guess I can notice my voice shaking or breaking, or the blocks becoming more pronounced. This causes my nervous system to try and stop me from speaking. The mechanics of this I read in few books is quite complex. But short version is that my sub-counscous is trying to keep me safe by evoking 'freeze' response in a 'scary' situation. This 'freezing' is what makes our stutter worse.