I strongly agree that it DOES matter why your horse is behaving this way. I appreciate the very systematic suggestions you’ve received to get to the bottom of the behavior, including eliminating all medical causes first and foremost. I also experienced Lyme disease as a cause. And I’ve found that ill-fitting tack can be a cause. (The horse tolerates the discomfort for a long while, but each rides builds reluctance to the point of “I can’t do this anymore.”) Once all physical causes are eliminated, a positive approach to retraining that includes rewarding good behavior can make her comfortable and confident in the cross ties again. Without eliminating the cause, a quick fix using discomfort to dissuade her puts more pressure on her and will create a more dangerous situation. Very best of luck.