I completely agree with the sentiment about the mental aspect being one of the biggest aspects of refereeing. Out of the last 2 years, my hands down worst games I have officiated were: 1. that day I have woken up early to bail my friend out of jail for a DUI after we had hung out together the previous evening. That weighed heavily on my mind (survivors guilt, I didn't think he had as much as he did) and later that day, the game I did (a U12 DA) had both sets of parents almost fight with each other, both mad at me, and a red card for an 11 year old kid. Horrible day. And the other terrible game I had 3 hours of sleep and left work having issues on Friday.
Both those games I was just as physically fit, and had the same understanding of the laws as I always did. But, it was entirely the mental aspect.
Also, on a more funny anecdote. I remember in like the 88th minute of a game I thought to myself "Wow, the ball hasn't hit me once this whole game, my positioning has been on point," not 10 seconds later, a kid whacks me with the ball because I was careless about my positioning because I was daydreaming about how good my positioning was. So even a temporary lapse in your mental can affect your game. Good luck to the referee doing his 5th game of the day and him trying to focus the full 80-90.