I wouldn't worry about how she moves in the round pen, it's very typical for them to move like that. It's also very natural for them to "motorcycle" around her turns, it's a lot easier than being upright and balanced. =]
As far as riding her goes, all I can say is half-halt, half-halt, half-halt! She can canter slower, it will take a lot of work to get her balanced enough to do it, but she CAN. Don't let her talk you into thinking she simply cannot canter any way but quickly.
And in case you don't know, a half-halt is exactly what it sounds like. If you are half-halting at a walk, two of their legs should stop moving. It's like you are suspending your horse's movement for just a second, and it gets them back on their hind ends. To aid for a half-halt, you use your outside rein, inside leg, and your tummy muscles. Tighten your outside rein, squeeze with your inside leg, and tighten your tummy muscles, all at the same time. It helps if you exhale when you ask.
Another thing you can try is using your breathing to slow your horse down. Really. I'm not crazy. It works. Start at the walk and ask for a halt. Walk along, inhale, and on the exhale, tighten your tummy muscles, tighten your reins, and think "HALT." It may take a few tries, but I guarantee your horse WILL hesitate in her step. When she does halt, ask her to back up a few steps, again using your breathing, to help drive the idea home. She will become all the more responsive in all gaits if you can use your breathing to steady her.