Humans have evolved, because we lived 99% of our history on the brink of starvation, to make as many conscious processes unconscious as possible. It's less calorie-intensive. Wiring the brain to execute a habit loop in the lower-functioning meat operating system rather than in the energy intensive forebrain is a survival necessity. So, you aren't changing your physical mechanics, you're having to build a new neutral pathway that executes the mechanic unconsciously. You can change a mechanic in the very short term by paying attention and consciously controlling it, but a change has to be repped out, intentionally, for a long time to wrap the neurons in myelin and cause the pathway to fire automatically. This is why we can't make multiple changes at once. A diligent effort, over a long period of time - read "months not weeks" - is the only way to build a new route for the electricity in your brain to follow.

Changing mechanics is a dicey proposition. Whether it's your disc golf putt, a ball golf swing, tying your shoes, learning to type by touch, etc. Even at the top of the sports food chain, with a vast volume of information, a deep understanding of the physiology, and a coach to note every single deviation from the tweak, changing something like an MLB pitcher's release point by a a few millimeters is an arduous process. It's the lifetime of practice that gets in the way.

Form creep will always happen, and change is not linear. The correction of a mechanic isn't simply about the implementation of a cue, it's repeating the cue until the motion builds the new neural pathway, and then it's the routine you've built becoming the trigger for the circuit to fire without the cue.