The biggest incremental jump in the 20 years I've been playing, happened about 7 years ago.
I quit CARING. I literally just quit thinking about anything during my swing except for just trying to make it feel like I was just letting the club's own momentum start and move through the downswing, and then just before the bottom get some nice easy snap on it. I can't really tell you what I'm DOING to produce that, other than it's pretty much like bracing your weight transfer with your front leg to accelerate a baseball bat through the hitting zone. I don't micro-manage my setup, posture, forcibly clear my mind etc. (in fact, I prefer noise/distraction from others while swinging.
So:
Stop caring about the outcome of the swing. (because that is the best way to screw it up in my experience)
and
let the downswing handle itself, and time the "snap" properly (don't rush it IOW) at the bottom. It's 100 percent feel, zero percent worrying about how the club gets there (square at the bottom)
I've never had a lesson, so my terminology is probably primitive/stupid sounding. One other thing, I quit playing as much. Used to, when I had the chance, I'd play sometimes three rounds a week. That frequency seemed to make me play worse. It would now appear, that I play better, when playing a lot less. Not sure what that even means. LOL