Mare's back sore...please read!?

That is a bitting rig teaching the horse how to flex and maintain a pace with the length of the side reins and perhaps an overhead chambon to keep the proper carriage of the head (sometimes they drop their head to low and tuck the chin)a crupper should be used to keep the rig center and not crawling up her withers. You do not belong on the horse in this rig! She is sore because you are not adjusting the outside rein to compensate for the arc of the circle to which she must bend her forehand to follow the circle. but her poor NECK! she can't properly turn due to the effect of the side reins!! She probably has the most terrible stiff neck! Just like you walking and having to turn the corner but your neck and head not able to lead you in the turn terrible! You must readjust the side reins when you reverse direction on the circle as well, if you are driving or serpentineing you can keep both sides equal, however the circle requires slack on the outside rein. Best of luck!