my advice will be ground work like really basics, I had a problem with a leased green horse at the barn to it was a torture, in the circle he used to do what you describe, sio we went back into doing it walking, few days latter troting we stop there for weeksss lol before beign able to do it cantering.
If I was able to work back this horse I bet you will to it tooke me 3 months to have him back on jumping cos we leased him and ths rider just frankly ruined him.
When we got him back, you started walking him and he was awsome then if you ask him to trot he will just stop in the middle of the ring for hrs and if you get pushy he will just start rearing over and over again until he gets you down, now he is back jumping lots and lots of ground work once you solve the neck and the circles thgen worry about the jumping.
Ours used to trot to the jum canter the last 3 steps jump and stop completely we had to start making him at least walk, then trot, now when he finishes jumping he tries to stop but if you say cmon he keeps cantering lol, so you have to yell after each jump lol but he jumps.
Its like starting all over with a brand new unbroken horse.