Here's a heels down tip that really helped me, TOES UP, not heels down. Like BSMS noted, it messes with your leg position if you try to cram your heels down. Instead, just try to lift your toes up a little bit, and I found my legs didn't move.
I'm 55, rode English all my life, showed and did all the jumping and stuff. Now, I'm all about low & slow, I started riding Western Pleasure and Trail classes 2 years ago and I'm loving it. My husband is just now learning English, and he's having a tough time too. Unfortunately, he's not real consistent with the lessons, so he's not having an easy time. We're hoping next summer he'll be able to start going 2X/week at least and then he'll see some real progress.
We all have to start somewhere, sometime. I still want to learn how to cut cows but I need to get better at the western stuff first. So....maybe when I'm 60? LOL! We have no plans to quit riding anytime soon, so just keep on going and keep on trying. I'm sure your instructors love you, you're never going to give them a bad attitude, get pissy because they take away your stirrups or throw a tantrum because your favorite school horse is lame. And you probably pay on time.