I would highly recommend stair lifts! What you do is find a set of stairs or something with an edge then stand with most of your feet hanging over. Let your heels sink all the way down, hold for two seconds then bring them all the way back up. I do that 3 times a day about 30 times each which is all I can handle before the pain is too much. I swear by this method for not only building muscle but also making my heels much more flexible and I've seen it recommended on various forums.
... also, when you're riding make sure you're not forcing your heels down! That is a huge no no but it is such a common thing for riders to do. You always want to let your weight sink down through the heels, naturally forcing them down. If you're having a lot of pain, I would look into flexible stirrups. I know a few riders who can only ride with them otherwise the pain is too much.
The hardest part isn't finding what we need to be, it's being content with who we are.