TL, I didn't watch the entire video, but watched it for a while in several places. What is her reason for cranking the horse's head around to the side while riding him forward? She was turning his head more than was needed to ride a circle. No wonder he's rebelling! I think you need to ride him yourself and teach him to go forward first. When you ride a circle, do it without turning his head more than he needs to. Then your circles can become smaller while his whole body is curved and not just his head and neck.
That's true. Horses develop at different times, like kids, this horse could still be a little immature and, his mind can't grasp the concept, then, being pushed, he blows.
TL..... sounds like a great idea, take him out of training for a while, let him just be a horse, then start slowly, in a couple of months.