So sorry to hear that, at the rescue we have a mare who had a series of problems, the least of which was a ruptured tendon. We have discovered that using a shoe with an extended bar off the back and a slight heel helps keep the pressure off the tendon while it heals. With our mare she healed up fast and well. Despite that and her numerous other issues she was just ridden the other day, walk and trot (and canter which was all her decision :P) by one of the younger volunteers. She was so proud of herself. She'll never be 100% again, but I think with time your horse will be a valuable rideable companion again. Probably not high competition level though or much hard work.
Hope all goes well for you, and thank you for being one of the few who truly has committed to your horse for life.