The book is, Golf Is Not A Game of Perfect by Bob Rotella.
Great book.
Another great book is In Pursuit of Excellence by Terry Orlick.
The mental side...oof.
If you figure it out, let me know.
On rare occasions, I've found a kind of 'flow' state. I can't offer a crisp definition of 'flow', but there's a Radiolab podcast episode about it that hits the mark. They describe an experiment in which participants had an electric current applied to their brain that induced 'flow'--incredible response times, complete immersion in the moment, statistically significant improvement in overall execution.
So I suggest you apply some voltage to your brain.
Practically speaking, I think every athlete wants to find 'flow' when their performance matters the most. But this often just results in a lot of mental clutter and self-talk instead of actual flow. It may even detract.
It feels impossible for me to declutter-at-will like this. I want to find that pure execution state of mind. I know what it feels like, it's almost like there's no pre-thought. That is, there may be rehearsal (e.g., visualizing a shot shape and throw) but the mind does not wander on stage and deliver a distracting self-lecture.
I'm going to try to shake things up myself. I know that analysis in moments of execution, and after execution, can be detrimental. I find myself on the course engaging in philosophy rather than something more like a hunt or a battle.
For some reason I think of Michael Jordan, when he would go off. He'd find an object or person as a focus of his vengeance and just use that as fuel. Not saying this is 'the way' but it's a useful case study.
This topic is fascinating, and I hope to add something of value through my own efforts.