When you learn any skill well enough to perform it without actually having to consciously think about every element involved (like riding a bike or reading these very words or carrying on a conversation), your brain functions in accordance with its "implicit" system of action -- basically it runs like a computer program, using memory (your knowledge/experience/muscle memory, etc...) triggered by realtime sensory input (what you are seeing, hearing, touching, feeling, etc...).
While learning a skill we have to focus on every aspect of what we are doing, consciously performing every action. This is the brain's "explicit" system at work. The more you do something with explicit control the more you develop the ability to perform the same actions automatically without having to explicitly control each movement. You've programmed yourself, and therefore the task has moved into the implicit system of brain function.
But even when you have enough pool playing experience stored into memory to allow the pool playing program to run implicitly, your brain is still using sensory input to orchestrate the entire process in real time. This means ANY unexpected or bad input can cause a glitch in the program, and when the program hangs up we find ourselves consciously thinking about what we are doing. We end up trying to control every movement with explicitness rather than allowing the brain's implicit system to do its work. The unexpected input can be something we hear (your opponent's mouth), or something we see (a shot or game situation we aren't familiar with), or our emotional state or frame of mind causing bad mojo in our head.
I know this is long-winded, but it all boils down to understanding how you are capable of playing great in the first place -- implicit brain function -- and then learning how to deal with unwanted or unexpected inputs that cause you to start using too much explicit brain function, which can make you feel like a beginner again. Many times, the best way to deal is to just tell yourself, "it's only pool". Go wash your hands, look into the mirror and tell yourself it's only pool, and stick to a solid psr to help keep the program running smoothly without overthinking or trying too hard.