My horse randomly started headshaking at 18 years old last spring. I was so worried that it was going to be a permanent problem from then on after researching the condition on the web. I bought the “muzzle relief net” (from Smartpak) to attach to his bridle to ride with and I didn’t really notice a difference. I was hoping it was allergy related since it came out of no where in the spring but didn’t want to just give up riding during allergy season. I was about to try some supplements for the problem when I decided to get my equine chiropractor out and he found my horse was terribly out of alignment at the poll. Two adjustments later and the headshaking went away completely! Thank goodness!! He has been fine since. I know it’s a long shot for your situation since it sounds like that horse has been a headshaker for a long time, but just thought I’d share my experience. From my research online, the muzzle relief net and some supplements have had great success with horses with this condition.