Hi Erin, have you ruled out that this girl is not back sore? given what you’re describing, and that it is a new situation, my first inclination is that as she is experiencing some sort of discomfort; either back or lower neck. The fact that she’ll trot fine on the lunge is another clue to me. When she trots on the lunge, is she stretching her neck down? Since this is a new thing with her, I would want to rule out that she’s not trying to tell you something before you do a lot of retraining. Good luck to you and her both.