This is slightly unrelated.
Because of my own disability, I wore braces on my knees for nearly four years. These braces went up to my knees, strapped, and came down beneath my feet into a molded form. These braces served the main purpose of making my feet move into the correct position so my knees would not hyperextend and cause more damage. The braces gave me the ability to move my foot up, however I nearly had to have surgery because I have so much restriction in moving my feet up.
For the first few months it was odd, painful. However, after the years, I finally learned how to jump, run, and do nearly anything. I felt more comfortable, more secure with this. At times I have considered fusing my ankles, however the idea frightens me and you are a very brave girl to have gone through it.
Everything will take adjustment, however I believe you are capable of doing it.
When you ride, you don't put your heel down. Especially in the faster gaits, if you put your heel down you will tighten your calf muscles and cause tension in your legs. Rather, relax your seat and try to bring your weight down completely. Let gravity move through your body, and let your weight go down, down, down, and relax. Your body should naturally move that way.
I may also suggest riding without stirrups for a little while. If you are capable of moving into the rhythm with the horse without stirrups, and get your weight in the correct positions, hopefully you can have a smooth transition into stirrups.
Something that I was convinced of when I was first diagnosed. I was having emotional problems. A teacher of mine told me something that I still hold to. You are capable of anything you want to do. Everybody is different. A blind person may be more capable of playing the piano or writing a story than a person that is healthy, just as Frank Hopkin's wild mustang may be more capable of an endurance race across a desert than the finest bred Arabians. It may take you a little bit longer than other people to accomplish what you want to do, but try to keep from excuses. Tell yourself that you can get past this, no matter how long it takes, and don't give up.
I don't think somebody with your kind of problem is a commoner around these forums, and you are an inspiration to me, and I am sure, many others here.