All jumping is is stride control. You can't successfully jump if you can't control the length of the stride. A horse can take a jump one of three ways:
1. In stride
A horse in a true frame and balance will be able to take the jump any of these three ways and still jump up and into your hands, have a nice bascule and jump from his haunches.
A horse that is unbalanced and not in a true frame will lose his rhythm if he has to take it long or short (get faster, break stride, jump flat, etc...)
Your mare isn't giving you her back what so ever at the canter, and when you say you feel like you are grinding into her back when you sit the canter, this is why. She's bracing against you, which is what most OTTBs do. I would ride in a deep two, and ask for inside flexion, outside flexing, inside flexion, outside flexion...until you feel her soften her mouth and back. Reward her by letting your reins out a bit and let her reach into your hands. Once she feels soft, slip into a deep three and keep your butt in the saddle and let it move forwards and backwards with her gait but not up and down. If someone put a cold hand on your back, you would hollow it. This is what your mare is doing. Your goal is to keep her back soft so you don't make her hollow it away from you when you sit the canter. Do these exercises with ground poles at the trot and canter as well, if she is in a true frame and ready to lengthen or shorten her stride she will be able to take the poles in-stride, long or short with no change in the quality of trot or canter. This will make your ride much better. She seems to really enjoy jumping, so I wouldn't worry too much about starting all over again.
Plus, jumping is mostly flatwork with some obstacles in between. If you don't have nice flatwork, you won't have nice jumping.