There's a lot to this. It's more suitable for a DM conversation. Why do some situations trigger a more severe stutter? A lot of its fear based. Even if you no longer actively fear that situation, your body has been programmed for years to react to certain situations, certain ways. It's the fight or flight mechanism. And your fear is telling you I am going to stutter here, more more likely anyway. Therefore, you should flee from this. I gradually became very accepting of my situation and the fear has decreased, but I noticed exactly what you describe. I may not be actively fearing the current situation, but my body responds very differently than my thoughts. I "lock up" as you say, and I become numb and lose control of any organized thought. Look up somatic therapy. Because of our chronic trauma our body has developed responses. it goes beyongld changing our cognotive thoughts.