First off welcome to the forum
Seeing how tense the horse is, I would go back a few steps and start off your work day with some lunge line work. Before you can ask a horse to stretch down and move forward from behind, they have to be balanced and get rhythm. I have a feeling that horse is not either. I would do some short sessions before each ride on lunge line and work on simple transitions while allowing her to move forward and relax her top line.