I really really really would strongly suggest holding off on jumping until you can get the canter up to par. If you are having trouble with "GO" on the flat, you are going to have even more trouble with "GO" over fences, no matter how small. Your horse is already telling you this, and is going to get into some seriously bad habits that will at best be difficult to break and at worse be downright dangerous for both of you.
Having said that, I completely sympathize with you. Getting horses to canter was probably the most difficult issue I ever faced in learning to ride ... and an obstacle that took lots of practice and many tears to overcome. It's not easy, so don't be discouraged. It will just take time. I've also spent years teaching beginning and intermediate riding, and cantering is by far one of the most difficult things for riders to learn.
I wholeheartedly agree with Spirithorse's suggestions. Also, I'd start by having your horse canter freely (without a rider) on a lunge line or by free lunging. If that works, try it with a saddle, and if it's still ok, try it with the bridle. This should be a pretty easy way to tell if your horse is comfortable and isn't being difficult from a physical issue.
If your horse isn't use to cantering in a ring, or in a specific space with turns, it can be hard for them to learn to collect and bend.
They key to getting a horse to canter is to set her up correctly and not punish her once she picks it up. Your inside leg should be at the girth, your outside leg should be back a bit, and your horse's haunches should be bent towards the inside. If you are asking for canter from a trot, make sure you are sitting and not posting. Keep your horse collected while you ask, and be sure to drive with both your seat and your legs (squeezing all your cheeks!
). If she starts trotting fast and gets strung out, you need to bring her back to a collected trot and try again. If she makes an effort to pick it up, make sure that you aren't catching her mouth or landing on her back. It's not helpful to lean far forward when asking for the canter(many people thing leaning forward will get them to go, but it prevents you from using your seat to drive), but riding in a half seat once she's picked it up will keep you lighter and her more comfortable while she learns her job.
It may be that she's still learning what it is you want and what exactly your aids mean, so be sure to encourage her for every little effort. Even if she only takes 2-3 strides, let her know that's what you wanted instead of just kicking and trying to get her to go further.
Has she cantered well for you in the past, or is she just learning? How well do you do with cantering on other horses? Has she cantered for anyone else?
Let us know how it goes with her. Wishing you all the best!