That Bruce Lee quote ya got there just may be the answer to your question.
I'm in the same boat as you. My stumbling point, unless I fall into that zone we love, is usually the mid 30s or 40s. I think that's typical for us amateurs.
There are simply too many factors involved in dissecting what goes wrong specifically. Usually it is more than one thing. We are able to fight off one or two things for a while and after a while they start piling up on us both in our minds and on the table, and we miss.
Subtle things go wrong, and the better we get, the more subtle the things are that get in our way.
The frustrating part is that while some of the things that hinder us are the same, or similar, many more are unique to our own game and thought patterns and envisioning of the shots, the feel and touch of individual shots and pattern requirements and one person cannot specifically help another with their individual demons.
I know I can't expect much more playing 5 or 6 hours a week. Maybe 5 or 6 hours a day may help a bit, but the best advise I can think of is to be VERY aware of what goes wrong and remember it.
Are you attempting a touch draw shot that is on the edge of your ability to control? If yes, remember it may not be the shot itself that gets you, you might make the shot and the position, but the change of tempo, rhythm, or thought pattern that triggers the conscious mind that may get you two shots later. Remember, these things are subtle.
Did you get a tad too straight on that shot in the upper corner pocket and now you have to force follow it two rails a bit to get back in line? You can do that, you've done it many times, but ..... it triggers conscious thought because while you can do it, it certainly isn't as automatic as that last 4" laugher hanging in the pocket. So .... we take a second, fall out of rhythm, and the decay process begins.
Not to mention that the 4" laugher may have been the cause of our getting out of line on that shot in the upper corner because it was so easy we lost some respect for position and got to casual with the shot. Now we pay on the next shot. Very subtle.
Subtle things that enter our minds are a huge hindrance to extended runs. So many ways to screw up and so few ways to be right on for extended periods of time. Especially playing a few hours a week. Don't knock yourself out trying to fix this, just play, enjoy the game for what it is and let the runs come rather than trying to make them come.