I feel your pain, nothing worse than trying to get somewhere and your just waiting to hear a bang from the trailer! You could maybe try short trips if youv the time to help them get the hang of starting and stopping and going around bends.. Am not sure if it would work for your horses but my granda had this same problem with his shire horse, you could actually see the trailer wobble.. now he had an ivor williams trailer without a partition, probably a different shape to yours.. but anyway ..he put 2bales of straw (sm rec ones) on either side of the horse and it worked a treat. I had to stand in the trailer once for a short trip and the horse almost lay his arse on the bales going around the bends lol so if things get desperate you could maybe try out something like that.. good luck