Autism

Pretty similar experience to what others have mentioned, so mostly echoing what they said.

With a shutdown I "hit the wall" so to speak. I could have been fine when things started out but I have reached the point where whatever it is has built up and I have reached my limit of overstimulation. I will stop participating in conversation, if it's really bad I won't even move much or be paying attention.

Because I grew up with "you're not autistic you're just high maintenance" parents I learned to "save" my meltdowns. I may have short responses or act annoyed when I'm around others. Inside I am raging. And I know I'm raging for no reason aside from the environment, but I can't help it. Which makes me more mad lol. Overstimulation is not just "wow it's loud in here" it's physically painful to me to continue to exist in that environment. Once I get home I rage cry.

Another thing that can make me shutdown or have meltdowns is plans changing, not only overstimulation. When I have plans it is like my entire life leading up to that point is built on the fact that I will be doing X at Y time. I have spent the time to mentally prepare myself to do X at Y time. If those things change, everything feels fucked up.

Recognizing when I am becoming overstimulated helps me a lot now that I actually understand what is happening. If I get to take a break and go for a walk or go somewhere quiet it usually prevents me from having a full on meltdown later. Also just... Admitting what is happening. Like "I am angry because the time changed from 3pm to 5pm and I was not mentally prepared for that." But some situations are just too much and it ends up happening anyway.