A few days ago I got really tired of it and decided I would work on it from the ground. For about a mile and half I had her walk beside me (no reins/lead or anything), trot when I said the word and sped up myself, and stop when I said whoa. By the end of the mile she would nearly be on her butt stopping as soon as I started to say the word. I also worked on her backing up at the touch of my hand.
What an incredible difference that exercise made under saddle! I can safely say we've had a breakthrough. I could transition from walk, trot, canter, stopping and backing up easily and with no resistance. It might sounds sappy but I was so happy I cried.