"It goes downhill" may actually be the answer. You have to help her shift her weight to her hind legs and to pick up her shoulders. Lots and lots of transitions between gaits will definitely help - and you have to aim for the transitions to be clean and calm, not rushing, choppy or pulling her to a slower gait - use your seat instead. My trainer taught me to go in a large circle and transition back and forth from trot to canter, keeping a balanced seat and a soft contact. If the horse tries rushing, turn him in a smaller circle until he slows down, then return to a larger one and praise for good, soft transitions. They keep their attention better if the transitions come every few strides, but don't overdo it not to overwhelm the horse. Do gait transitions every time you ride and be consistent with aiming for responsiveness.
I have come a long way, to surrender my shadow to the shadow of my horse.