Bit of background, 7yo cob only broken in last year. I’ve owned him 11 months. Coming on nicely over winter in walk, trot and canter and could do a nice intro dressage. Canter transitions were just starting to be good and we were getting on well.
Had a few issues a few months ago with bolting, bucking and rearing (long story short but it wasn’t pain related... just idiot people I was hacking out with not being respectful and bombing off without prior warning sending him over the edge).
Anyway, due to all this knocking my confidence I gave us both a month off ridden work, he had two weeks in the field then I started a bit of ground work and long reining him etc. Ridden work started again a month ago. But he’s gone completely backwards in the school! He can’t/won’t go in a straight line, he falls in massively on corners and when trying to get the correct bend his head pokes the outside of the school and his shoulders fall in and goes sideways and drifts 
It’s driving me insane. I do circles, slowly spiral in and then out, I do lots of 3/4 line stuff, going straight and doing 10 meter circles off the line then join back down the 3/4. Walk is just about ok but trot is so frustrating. He just won’t bend round my inside leg at all. Even on circles he drifts and leads with his shoulders.
He’s very good at lateral work, but he’s almost using this as a get out (if that makes sense)... struggles on a circle so just let yields himself out. Can’t go in a straight line so does his own shoulder in 
We have weekly lessons, his back was done last week because of this but there’s not been much improvement, saddle has been checked, teeth are up to date etc.
I do carrot like stretches with him after exercise whilst his muscles are warm.
I generally school 3 times a week, lunge twice and hack once. Or if I’m lazy school twice, lunge twice and hack twice 
Help! What can I do to improve this?
I’m on a big yard with a few riders by trade, I asked one to get on him to see if it was me, and he did exactly the same. So whilst it could be me, he does it with someone else on him as well.
I’m starting to get frustrated, what am I doing wrong 