Advice from a (mostly) reformed knees-and-toes-holding-onto-horse-for-dear-life rider: ride without stirrups esp. at canter until you find the balance and figure out how to not hold yourself on, post at the canter until your legs are going to fall off and then go for another lap, stand on your toes for a lap or two every ride so you have flexion in your ankles again (It is possible to have your heels too far down which is almost as bad as not at all down.), and put your stirrups two or three holes too short because it will show you when you grip with your knees AND teach you to balance all at once, and turn your toes out to a 45 degree angle with the horse.
I switched trainers two years ago from one that also preached toes in and spent the first three months doing nothing but flatwork on this exact issue. The above exercises are what worked for me, and I hope they help you out some too! Oh, but don’t try them all at once please! (I expect trying to post at the canter with stirrups two holes too short while standing on your toes and trying to turn your toes out just might be counter-productive! :wink grin:)
Good luck and happy rides!
ETA: When your heels are too far down (I’ve heard it called hyper-extended?) they don’t anchor your position like they should and they cease to act as shock-absorbers. You can, in fact, ride with an otherwise correct and effective position with hyper-extended heels, but it will cause you to lose feeling in your toes, etc. and it is more difficult to do so. I’ve been told also that it gets to be painful when landing from big jumps (grande prix, etc.)