Indirect rein in front of the withers. At the walk and trot, in the corners, hold your outside rein steady and bring your inside hand up and across the neck in front of the withers, and ask your horse to move to the outside a few steps in an arc. Use as much inside leg as gets it done. When you ride the short ends, turn early, then side step as above over to the rail. Continue with a figure 8, first direction with a direct rein to the inside, then cross the center, move the inside rein over the neck to become an indirect rein in front of the withers, inside leg hard at the girth, and move them around the second circle in a counter bend. Practice this until the circles of the 8 are the same size - first bent with the circle, and the second counter bent. Both directions.
When you can do the above, canter and practice moving horse into the center of the ring and then outward with an indirect rein. Spiral in and spiral out. Every time the shoulder drops, pick up your inside rein across the neck, (keeping good contact with the outside rein) and move your horse to the outside to lift the shoulder. If horse cannot do that by then, go see the vet.