Well, this may be a little different, but I have a Saddlebred mare, 6yrs old, who also has some difficulties with the canter. She doesn't buck (at least she hasn't with me, she may buck though if a rider continued to push her)... but she's obviously off balance and struggling.
She can and does canter on the lunge... tacked or not - but it's only recently been becoming more balanced.
Now... I am talking about a breed bred to TROT (and this little mare can trot any way you'd like it!) and it's not unusual to have to spend some time creating balance within her breed when it comes to the canter.
That said, I wonder if your mare is similar to mine... and just struggling with baby balance. My little mare keeps growing, not huge amounts at a time, but enough that she's unbalanced one day, and caught up the next... etc.
How does your mare canter on the lunge? Is she still on the forehand, or does she travel round? Are her transistions clean... or does she tend to "run" her canter? Have you taught her a vocal cue to go with the canter?
What I've been doing with my mare (other than the lunging to help her balance herself) is working on a lot of transitions... up and down, without using my reins... this helps her balance. She is NEVER allowed to run into the canter - if she starts to speed up her trot so she's running it, she is asked to do a circle, half-halt, or change direction until we have balance again.
Another thing to consider is how much cue you're giving her. Are you removing pressure the instant she has responded to you? Are you balanced enough as a rider to help set her up, or could it be possibly you're off balance enough to disrupt any balance she does have.
I have other suggestions, depending on your answers.