Something that can help is to put a strip of duct tape on the horse's shoulder so that she can spot the movement easier until she learns how to "rise and fall to the wall". To help her learn to feel the horse's feet, try at the walk first. Have someone on the ground call a foot (say, right front), and have her say "now" when she feels that foot hit the ground, and the spotter validates her guess. When she can name each footstep at the walk, move to the trot. She'll be feeling for her diagonals then.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown