We have a mare who whilst schooling drifts to the left, prob due to stiffness, which aids work best to solve this problem.Hoping to do dressage test in near future and need straightness.
I find using the outside aids helps with this problem. Use plenty of half halts with the outside hand. Try not to pull the horse around with the inside hand as this will cause her to drift. You can indicate direction with the inside hand but try not to pul back. Hope this helps x