If a horse is not ready to give you his back, you will always have problems sitting the canter. This just causes discomfort to both horse and rider. If you start out the canter in a deep two and do head and neck bending, you will feel the horse's back relax and his pace should slow. At this point, sit on your butt, not your crotch. And let your seat follow the horse's motion front to back. Just a pointer, that this will be extrememly difficult to do if you ride in a close contact saddle, as the flat seat of the hunter ring is not meant for finding your seat. It took me months of thinking i was a bad rider to figure out that:
a) I was pissing off my horse by trying to sit his canter while he was hollow backed
b) my brand new expensive close contact saddle was good for NOTHING when it
came down to learning how to use my body correctly
c) I needed a new saddle (enter beautiful Kieffer Aachen All Purpose saddle, that
gives me a great position on the flat and over big stadium and xcountry jumps)
d) my horse actually didn't canter like a 3-legged moose with a drinking problem,
he just needed me to ride him correctly.
If the horses you are riding have hollow backs and you try to sit the canter, it's just like putting a cold hand on the small of someone's back, they are going to shrink away from it and it won't be comfortable for either two of you.
Since you haven't been mworking on the canter long, talk to your trainer about letting you master your seat on a horse with an easy canter to build up your seat, leg strength and balance. Then once you get the feel of what sitting a canter will feel like, you will be able to ride a horse with a difficult canter. Good Luck!