First get her to stop bucking.
Figure out why she bucks, there is a reason. Is it a respect issue? Or is she in pain from teeth, saddle, back? Is she tying-up, or colicing? Do her feet hurt?
If it is a respect thing... As in she is just doesn't want to, then I would do some ground work with her. Then, do some circles, and backing up when in the saddle. Any time she bucks she must do something she doesn't want to do like small circles and backing up. If she won't do this from the saddle, then more ground work is needed.
You must become alpha mare, not her. If you tell her to do something, she is to do it. Otherwise it can get dangerous. As in bucking, rearing...
Once you resolve the bucking problem, you will be able to get her to canter. However, from your description, I suspect she has a medical problem. Like, arthritis in her back, or hips, or hocks, or fetlocks... Thus it is painful to canter with the back part. You might want to have a vet take a look at her. There are medications that help with arthritis, and other things. Also is there a chance she has EPM? It can cause problems with the hind end.
Will she canter in a round ring, or on a lunge line, or corral/pasture?
My horse bucked a lot. With the vet's help, I have found out she was colicing as a form of tying-up. When she coliced, she bucked. She also got into the habit of bucking, and she has always like to buck, as well as not being balanced on her front feet which caused back pain. So we are really working on this. She is now pain free, her front feet are shod and balanced, so lots of ground work! And so far.... No bucking!