Zigzag I would find it too emotional to keep seeing the horse if it belonged to someone else, so I wouldnt want it to be nearby. I think its a question of really, really vetting the buyers and make sure you only sell to someone you like and feel is right for the horse.
I would also make the buyer sign that they will give you 1st refusal if they ever sell. That way you can make sure you the horse doesnt get sold on to anyone else without you knowing.
Its horrible having to sell a wonderful horse and very upsetting, but as long as you get a super home for the horse you will get over it in time.