If I may interpret the question posed to read, "What is the major difference between those who can play pretty well and those who can't?" I'd say....
The lesser player doesn't choose the right shot often enough, and even if you tell them the right shot, often they won't know how to execute it.
Hence, the better player is playing with a stacked deck of cards.
[edit-added]
Another factor which I consider to be of major importance is 'working harder at the table'. By working hard, I don't just mean pressuring oneself, I mean making a hard effort to remained focused on the myriad of problems that occur, as they occur, and making clear and calm decisions for each of them. Such as choice of shot, method of positioning, adjustment of aiming, speed of delivery stroke, placement of stance and bridge.
All of these things can go wrong, and usually there is some nagging doubt about one or more of these aspects on each shot. You needen't run a checklist, but be aware of these nagging doubts as they occur and decide how to deal with them. e.g. You may be stretching for a shot and considering using the bridge. Make the decision which one you will do and be satisfied with it. Don't be thinking during the shot that you'll be p*****d off if you miss because you should be using the bridge.
At the point of delivery, you should be satisfied with your shot decision and aim, comfortable to make the stroke, not clouded with nagging doubts. That takes a lot of work to do on every shot. More for some than others. But if you work more effectively in this regard than your opponent, the wins will come regularly.