Can you find a way to talk with other riders with disabiliites? They may have some intriqueing suggestions on how to get around some of the limits you have.
Also, Nbbles is gorgeous, and though she may not be developed as a riding horse to her full potential, I doubt she feels that her life is less valuable or interesting living it as your "pet". All that matters is stability, companionship and lots of good food.
Come to think of it , that's what matters to me! Especially the good food part!