is the horse in a place where you can go visit him/her? that would probably help, if the horse's owners will allow you to come groom him/her. also, do you have photos of you with the horse? you could make a scrapbook of these pictures, and by doing something about the horse, it would almost seem like you were thinking of it/with it again. but the best "cure" for this "sickness" is probably going to visit it