Dressage that made Duke quite quite sweaty

Duke and I worked with John on 10 meter circles, bending, leg yields, and half halts. At the end, Duke was insanely sweaty. Even his face. I wasn't.
He chewed on the bit and tried not to bend. John said "I know it's frustrating; it's frustrating to watch." He also said that the stubborn quality that made Duke refuse to listen to me Saturday was what makes him good at cross country, he'll stubbornly try to get over the fence.
It was - just changing the aids and trying to get him to respond. It was - John having to tell me every aid every step.
I could feel when he lifted his shoulders and was balanced; I can feel when he rounds and steps from under and up and over. But I can't feel why sometimes he does it and sometimes he doesn't.
John said no more group lessons; I get too worked up.
It was a really intense lesson, but I just don't know how to break it into little parts and describe it.