Several years ago my assigned lesson horse was a downhill, VERY thick jowled, downhill back, low set necked aged “lesson horse” who had, at most, pretty bad western training (probably more of the blame goes to his conformation, not Western training.)
I got this very resistant horse to become light in hand for downward transitions by only applying the hand aid ONLY when the horse’s head was coming up, and then releasing the aid (advancing my hand maybe 1/4 to 1/2 inch and relaxing my fingers) and not re-applying it until the next time the horse’s head came up. Even at the trot there are two times each stride when the horse’s head comes up a little bit. If I applied my halting/slowing down aid at any other instant in his stride he would set his thick neck, take advantage of his low set neck, and expect me to carry his head while he went where he wanted to go at his preferred speed.
Using this timing of my rein aid after a few months he would slow down when I twiddled my rein finger. He also became a true pleasure to ride.
It sounds like your daughter is doing well. I would be very proud of her too.