I have retrained 2 with muzzle above the wither. First western trained so no idea and bucked. I lunged in side reins as you have. Reins too long. Nothing happened the first 4 days. The 5th day an inkling and she started to drop. I didn’t have to shorten the side reins. She figured it out herself. They were tightened a couple of holes weeks later.
The result - a dressagechorsecwith no buck.
The 2nd a school horse and I HATED her. Head up and not one millisecond of give in the entire ride. By ride 3 I realised it was a case of turn her into what I liked or hate every ride as this was the horse they were now putting me on. SIGH.
I started with standing beside her and while waiting to be mounted, as the horses are already tacked up for the lesson. I asked her to drop her head and had taught her that Good Girl is a good thing. When she did really well she got a rub on the neck.
So in halt I asked her. Good Girl and neck rub. In walk she got it. Good girl and rub. That was all for first lesson.
Next lesson. She now had it in walk. I have her going in walk correctly and gently encourage almost like a half step to go into trot but if tries to raise head walk and uh uh. Stays with head correct and Good Girl and neck rub.
So really the secret was to say Good Girl.
The result?
That was in a group lesson with my instructor’s partner. I received a text from my instructor saying that he had seen how Clover was going and would I like to have solo lessons on her in the outdoor arena with him instead.
WOULD I? All my Christmases came at once I started having individual lessons on her and she became my dressage mount who I LOVED to have a lesson on.