First of all, your horse is maybe hurt. Have you seen an osteopath ?
The, does your horse do it walking, trotting and cantering or only trotting and cantering, or only cantering ? I think that the most important thing to do is to make sure to work with him very calmly (nothing of hitting him with the crop and letting him get mad), maybe starting with working walking. You have to master the circles and the turns walking before thinking about mastering them trotting and cantering. If you already have a good level and your horse too, it might be a momentary problem. Maybe if you work the basis one time very calmly, then the problem will go away. You have to show him that you are the boss but alway remaining very calm.