Gotta love that canter! This may sound obvious, but is she really driving forward? I know when my mare starts to get lazy in her canter, its gets jarring and very hard to sit. I find that half-halts really help smooth things out-they just get the horse back together, collected, and under you again. If your horse "runs" in the canter, they're a god send for that as well. Its tough when you feel like you're getting bounced around like crazy, but remember to relax. Tension in your own body will translate to your horse and get them tense too, which will make the canter feel more jolting. Make sure you're sitting upright, and not leaning back into a "water skiing " position as you keep your seat active and maintaining the motion of the canter.
Did I mention half-halts
? Based off of my own experience, when the canter is hard to sit, its because my mare is too slow, she's 'running', or she's stiff in the back (something she is prone too)
I've spent so much time learning to sit Raisin's canter and its still not where i'd like it to be, so I can say don't give up hope! It will happen :)