For your heels, if you stand on the edge of a stair and allow your heels to drop and then push down just until you reach your edge back off slowly and do it several times a day only 6 or 7 times. Itll help stretch the Achilles and make it easier to keep your heel down.
Try to attach a spur strap to your stirrup then to your girth and ride with it on like that for around 2-4 weeks and then see how your leg position is and try to shorten your stirrups a hole to try and keep your heels down
To improve overall leg position, no stirrups! It's hard but it's so worth it! Also riding in 2point at a standstill, walk, trot and canter. As for keeping your heels down, practice everywhere! Practice on your bike, if you're standing on a fence or stairs, lean back on your heels. When you're in the saddle, go around in two point with your heels down. It will soon become second nature :)
Hope this helped,
Last year I started off doing no stirrups for 5 minutes a day and no I can go 30 minutes straight, no stop, no stirrups posting trot. It REALLY gives you a lot of strength, balance and improves your leg position.
Just make sure you don't post to fast! Keep it slow, it makes it easier!