I would love to do more teaching, so y'all c'mon.
Allie, you and your new gorgeous horse totally need to come visit.
I think he needs to go hacking out in the Virginia country side as part of his foundation. Of course, my horse's stride is roughly 1/2 his (and the guest horse's is a 1/3!) , so it would make for an interesting hack.
I'm going to take advantage of insomnia and type the rest to you now while I won't be interrupted.
First some imagery: Imagine your leg is a board that your trying to secure to a wall/your horse's side. Now imagine putting one nail in the board, close to the top. That's like gripping with your knee. You apply ANY force or pressure to the board, and the bottom of it (your lower leg) swings. Now imagine putting another nail in the board, towards the bottom, by your ankle bone. Two nails in the board, it's not going to swing, right? So while you're riding, if you feel your leg come off, think to yourself - put another nail in the board.
I really like your saddle, and I understand you're not in a big English riding area, but I'd love if you could find one of the older, no knee roll, flat as a pancake, close contact saddles like a Prix de Nations or a Prix de Saute to ride in for a while. A deeper seated saddle with knee blocks allows you to get away with pinching with your knee in a way the old style close contacts just didn't.
Okay, more exercises -
Start with the first stretch I gave you above. When you find your good balanced two point, do a "rider's pushup" - slowly close your hip until your chest touches the horse's neck, keeping your back flat and eyes up. Really focus on the body mechanics - as your hip closes, you must push your buttocks to the back of the saddle so your crotch stays over the middle of your saddle. If your pubic bone bumps the pommel, that's an "Ah! Ah! Ah!" reminder - push to the back of your saddle. This is WAY harder to do correctly than it sounds. Come back up to your balanced two point without using your hands or pushing off against the neck. You will have to both relax your knee and put that other nail in the board and secure your lower leg to do this correctly - your leg moving or shifting is another "Ah! Ah! Ah!" Do 3 - 5 of these at a time, really focusing on the mechanics, and then work up to several sets of 5. You're trying to make a new muscle memory to replace the old bad one, so it's not a case of practice making perfect, it's perfect practice making perfect.
When you're pretty solid at the halt, try at the walk, trot and eventually the canter. If you feel yourself getting sloppy, losing your lower leg, rounding your back or tipping forward onto the neck, go back to a slower gait and fix it.
After you're pretty comfortable that your joints are working correctly at trot in two point, here's one more exercise to create the correct muscle memory - pick up a trot on the rail and count 5 strides sitting, 5 strides in two point and 5 posting for one lap of the arena. Pay attention to what happens to your hip, knee and ankle as you change positions - really pay attention to the correct fold in your hip as you move into two point, and opening your hip as you go to sitting. As this gets easier, add a lap of the ring at a lower count - 1 lap 5/5/5, 1 lap 4/4/4, 1 lap 3/3/3, etc.
Then it's time for the no stirrup work, and some grids and gymnastics, trying to carry this new muscle memory over from the flat work to the over fences. And I say no stirrups not just because it's a terrific way to build strength and fitness, but because it's much, much harder to brace or pinch with your knee with no stirrups, so it's easier to find the correct mechanics over fences without them at first.
Don't feel bad - for a long time in my dressage education, I could only sit a working trot correctly without stirrups; if I picked them up, I reverted to my hunter seat leg and pushed my seatbones off of the tack!
Good luck, and let me know how it goes!