I used to have a share horse who was a headshaker an IDxTB. It was a summer thing with pollen. She could not help it so I rode her on a long rein so when her head went she did not get jabbed in the mouth and I did not get whiplash! The owner for a nosenet for the bridle and that helped. Before she was semi retired and I shared her the horse was a hunter and had the summer off so the headshaking was never a problem. Hopefully you will find a solution to your problem.
