Also, if you carry your hands very high for the transitions it's impossible for her to buck if her head is u
Use MARE MAGIC. It's an herbal supplement that helps with crankiness and aggravation in mares which may be causing her buck
Ask your trainer they might have some times. Be prepared if you know that he will do it sit deep and push him on a horse won't usually buck if you're pushing him on. Horses can also sense your emotions. When transitioning talk to him a bit when you ask for the canter make sure you do deep slow breaths and that will help your horse and yourself relax!
You might try doing trot and canter transitions on a circle. Trot-pick up three strides of canter-back down to trot-repeat. Make sure you don't let her drop her head because this makes it easier for her to buck. Doing lots of transitions and praising her when she doesn't buck should help a lot.
*also might want to make sure it's not a health issue such as stifles sticking. this could make her want to buck to "unlock" that joint.
This should help with her to learn that the bucking behavior is not acceptable. Eventually she will just transition with no problems! Hope this helps ! Good luck!
When she goes to buck stop her instantly and back 10-12 steps. This shuts down the bucking. Then ask again and if she does the same thing, you shut it down again.