You look like you have quite a strong upper body, but it is clear in all three pictures that you are pinching with your knee. This has caused your lower leg to slide back and your heel to come up. As others have said, remember to drop your weight into your heels and use your calves as your base of support. Sometimes when schooling, it helps to remember to turn your knee off the saddle so that your calf is holding in place on your horse's side. Don't do this excessively as you do not want to have your toes sticking out, but it will hopefully help fix the knee problem!
In the last photo, I would like to see you open your chest up so that your back doesn't round, but judging from your other photo's, I think it was just a fluke.
I do like your long release, your FAB. upper body and how you manage to keep yourself balanced over the seat without a good base of support! Overall, not too shabby :) Keep up the great work.
And gorgeous horse !