When a horse cuts corners, the natural respose it to pull your put outside rein and push the inner one against the horses neck. This makes your inner shoulder collapse and your body turn to the outside. You need to reverse this. Lift your inner shoulder, sponge the inside rein slightly away from the neck, and keep contact with the outside rein against the neck. Bring your inside elbow to your hip, not your hip to your elbow.
By lifting your inner hip and shoulder and bringing them back, you free the horse to move around your inner leg into the corner