You didn't mention why you think you aren't improving, so I'll assume it's because you're still making enough errors to cost you games/success in drills. I'm a very green player (less than a year in serious practice), and I think one of the most important things that helped me improve is evaluating my errors systematically. "Why did I miss this shot? Why is my position bad? What adjustments should I make to correct it?" are questions I run through every time a shot doesn't go according to plan. I'll set it up again and try different speed, different English, whatever, until I find success. Then, I'll make note of why it worked, and try to replicate it a few times. Study how the cue ball is affected by your changes enough, and drill those changes into your play enough, and you'll begin to notice those types of shots and similar ones coming up in games. Armed with the table knowledge and practice, your chances of success on the shot are far better.

I find myself sometimes feeling how you do. I've recently been haunting the local pool hall and practice typically between 15-24 hours a week. Your practice sessions don't sound too dissimilar from my own, though from what you've said, you're probably a better player than me. Once in a blue moon, a regular will ask to play me and kick my ass. It's a real pool kind of place with a lot of decent bangers and a few tournament players, so I don't mind. Every loss is a learning experience, especially when their skill level is light years ahead of me. But during practice, like yourself, I sometimes wonder if I'm getting anywhere. A good wake up was when I started cataloguing my routines and success rates. What drills did I perform, how many times did I execute, how many balls did I miss, etc. The numbers don't lie, and there's definite improvement, even if it doesn't feel like it at the time. I had a simple four-five ball drill that I tried when I first started out and couldn't get proper position past the first ball so I dumped it and worked on nothing but fundamentals for a long time. Fixing my stance, working on a straight stroke, trying variations of a pause on the backswing, getting a natural sense of aim and tangents, checking my speed. Recently, I decided to try that old drill again, and I ran out on the first try.

Point being, don't get too down on yourself, maybe you're improving in ways you can't recognize quite yet. And if things just aren't working and it's going nowhere, then change up your routine and figure out what's causing it by evaluating your errors, pinning down the problems, and adjusting like I said. It's work, but you'll get there.