When your riding, keep working that trot and make her a little more balanced, do this by changing the rein often, circles, trotting poles, figure of  8, weaving, and some turning. She will be able to move better in trot and to work the canter, lunge her and get her to canter, it dont matter if its a couple of strides, she did it, and will slowly be able to build up, each time she does it give her a big pat and tell her shes good. But if she bolts stop her straight away, cause she might get the habit of just bolting instead of cantering. Then try free schooling a get her cantering and again it dont matter for how long, as long as its steading and comfy for her (Not A Bolt!!) But make sure each time you ask her to canter when lunge or free schooling make sure you say "Canter" and "good girl, come'on, CANTER!" then when she is cantering tell her "good girl" and say that when she's done aswell. The when you ride, on the corner ask for a canter and ask with your voice to, if she dont keep trying the lunge and  if she does, dont worry if its to strides, just build it up, but dont try to much cause she will get bored and wont do it again. :) xx

Try lunging or free schooling (with out any tack, just shoo her round the school with a whip)