tl;dr how do I make my brain bigger

I try to be as calm and collected as possible whenever I get into an upsetting situation or I hear upsetting things, but I'm genuinely powerless from going into fight mode in these types of instances. Like I can consciously feel my brain turn off my thinking go into chimp killbot mode. In these moments I have to resort to just not making a sound and removing myself, or if I'm able I just switch the topic as quickly as possible and try to clear my mind. I would much prefer that instead of this happening I could just be indifferent and not become emotional at all, despite being troubled by whatever is going on