My first thought is to question whether you're getting adequate sleep. Fatigue is probably the single biggest driver for huge swings in mental performance, for me, personally.

No that's perfectly normal. There are a lot of factors impacting how well someone brains at any given moment - for example hunger, tiredness, simply their body-clock, if they're on a roll etc. People who are switched on permanently and who never make dumb mistakes don't exist and I think one of the missions everyone has into adulthood is to work out how you work best and try to adapt to flow with that as much as you can.

Otherlad is definitely right about sleep.

I will also add to that socialisation. I notice the times where I've been isolated for a few weeks without seeing anyone and only having token interactions with cunts at work, and especially when I was trapped in a job where I didn't actually like any of them, my thoughts just increasingly became incoherent and cluttered, like just slowly coming unravelled. Then after getting to hang out for a few hours and spend quality time having meaningful conversations with a friend, I would feel recharged, literally like a fucking Sim having the bar refilled.

Depression can do a number on you in terms of cognitive ability, also. I spent a great amount of the last year or two being probably the most catastrophically depressed I've been in probably my whole life. I've recovered and made a lot of meaningful improvements over the last year or so, and that has really felt like a mental fog clearing away, like you don't even realise how badly it was affecting your ability to think until you come out the other side.

It can be a lot of things really. Are you getting your 5-a-day? Eating plenty of fish? Are there any women causing unnecessary drama in your life?