I had the same problem with rocky when he first came. He's been ridden in a flash and standing martingale with a pelham and tight nose band. He had an upside down neck and went with his nose in the air.
When I took him out in just a plain snaffle. If I asked him to slow down or to stop he stuck he would stick his nose out in front, open his mouth and leave me with nothing in the way of control.... so I understand what your going through. All horses are different so its going to be a case of trying different things for you and horse im afraid.
When I started rocky I used to hold my hands wide and low [ because im old and thats how we were tought to ride a puller in bygone days

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It was a case of give and take, not constant pressure on the reins, half halts, and loads of transitions while we were hacking out, until he forgot to be awkward. I think it took me about two years to be able to ride him out and trot along with him in some sort of a shape, without his nose stuck out in front.
I had the tooth woman look at him, and she said that he would have always have a problem with bits because his mouth was so small, there was such a small space for the bit to go, and he had a wide tongue to boot.
He went best in a plain loose link lightweight snaffle... [ ive got his if you want to try it 5 1/2"]
We cracked everything but canter, which was a problem that remained until he died. sorry.... but he never did canter happily no matter what bit I used or how I rode him... Opening his mouth was a real problem, I had loads of comments about this problem from dressage judges. So im no help, but I so do understand.
Have you thought about getting someone who knows about bits and bitting to come help you.