This is a difficult situation, because being tense/holding your breath/being stiff makes it impossible to follow the horse's motion at the canter, and not being able to follow the horse's motion makes you much less secure. So you really need to figure out a way to break this cycle in order to get your confidence back.
Is there something else you can do, within the limitations of your school's program, to get more comfortable? Ride a different school horse or take a private lesson? Are you working on riding the canter in full seat or two point? If in full seat, ask to have a grab strap put on your saddle for a while, so you can pull yourself into the tack and regain your following seat.
Are you familiar with any breathing control or sports psychology exercises? These can really help break the cycle of building tension.
Please do post more details, I would love to help you work this barrier.