To Sell or to not to sell? You pick!?

From what I can gather from your post I would advise against selling your horse at this moment, you have developed a bond with your horse, you like your horse, you don't want to sell your horse. It is you instructor that wants you to sell. you have a good thing going for you right now, and with your health issues it would not be good to change your life right now. Maybe next year.