Your mental and physical capacities are inextricably linked.
At a practical level, we already know this. You can tell when you had a bad day at work and you miss that max squat.
Or, someone cut you off in the parking lot and you hit a PR.
Or, you’re on a run and everything seems to just flow together. You get in the “zone” and then when you wrap your run-up, you realize and that was a PR.
But, this is also shown by science. There was a study done on isolation exercises. The scientists were showing 22% increases in force output when people focused on their muscle grouping being worked over those that didn’t.
And while that didn’t apply at above the 80% one rep max (probably because you have to go full retarded). It does show how linked we are mentally and physically.
In other words, if you want to get your gains in order, get your life in order.