How to manage left hand better

Hello! I learned that I should hold my racket with my left hand after the swing to ensure my left hand keeps up. As soon as I lose focus, it falls down, usually when I try to hit harder. In games, I usually forget to keep watching the ball long enough, and other basic things; I don't keep my feet moving, etc.

How can I manage my mindset to master my left hand (and other basic things), or is repetition and muscle memory the only way?

Here is a video from yesterday of my first training session with my brand-new Wilson Shift 99 with Solinco Hyper G soft strings.