How do you remember every instance?

[Dement]

Like others have said, there's a handful of mechanics which occur in tons of different fights, they just look slightly different. You develop the muscle memory to handle them and you develop the ability to recognize them rapidly when they start to manifest. "Did something fall and is still sitting there? I'm going to have to run behind it at some point" "Don't stand in AoE" "Aggro and kill adds" "Look away if you see an eye appear on a boss" "I have a tether, pass the tether to tank, creature, glowing thing" "Giant yellow arrows pointing at someone = stack" "Primal disappeared? Probably a dive of some sort, look for them now"

Anything that's really unique tends to stick in my mind for a long time simply because it is unique.

[Doki]

Relic/Anima quests have permanently burned most dungeon runs into my brain.

[SendohJin]

if you forget you forget, you die to it one time and it should come back to you.

[Khubla_Kha]

It'll come to you with repetition as with anything where doing the same thing over and over. If nothing else muscle memory normally saves me when normal recall fails.