system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can't someone just calm down when they are having a panic/anxiety attack?

I know that telling someone to calm down in such a situation is useless, and it usually will just make it worse, but why?  Even if nobody tells them to do anything, why is it so seemingly impossible?  I ask because I've never really had a problem with anxiety; when I'm stressed I typically just begin feeling tired, so anxiety as a whole is pretty foreign to me.
gpt
Panic attacks unnecessarily trigger your fight or flight response and send adrenaline all over your body. There is no danger, but your brain has erroneously perceived danger. You can't stop the adrenaline that's already been released in the same way you can't tell your brain to stop making your heart beat, essentially. 

Since it takes time for the response to die down, any attempts to calm oneself that aren't quickly successful can make the situation seem more endless/hopeless.