Warning: this post contains references to WoW. Do not be afraid, I will use as general terms as possible.
One of the things I say sometimes about having mental illness, and knowing you have it, is that you know how it affects you. You know that your emotions can't be trusted. There's this cognitive dissonance where you're feeling something, like anger, but you have no idea whether it's justified, if it should be more or less or at someone else or at yourself, whether it's an actual emotion or if it's those wonky chemicals in your brain making things misfire.
It's the same now. I was healing a Zandalari dungeon with a group of guildies - it's a hard dungeon and you need good gear and good awareness of what's going on, though it's not a raid, which has more people (10 or 25 instead of 5) and even harder bosses. As healer, I have to react quickly and be aware of everything at once - what bad stuff is on the ground, where the other players are, what the boss is doing and what the boss is going to do, what spells I have available and which I can't cast yet, how much mana I have and what ways I have to get more, etc.
It was like thinking through a layer of dirty cotton wool. I could see that my reaction times were slower, not like my body was moving through thick air that slowed it down, but like my thoughts were. And every time something went wrong I could feel the frustration surging up. I have done this before. I remember doing it. I remember it being, while not easy, easier.
That is, basically, what quake brain is. Everything is harder, but you can remember when it wasn't hard, which is frustrating, and frustrating makes you get emotional and strung out and want to cry.