Camryn, you need a good reason to canter. Have you ever had to move a bunch of cows and you need to keep one from breaking away from the herd? It's necessary. Imagine a reason for cantering.
It also sounds like you are clutching forward in fear. Try riding bareback for a while so you can learn how your legs keep you on top of the horse. Arms out, airplane style, rotate one arm forward and one back, hands on your head! Flex your abs, squeeze your legs, pinch your butt! Do that on the lunge line if you must.
Finally, there are so many good horses out there. An ex-racehorse is not one I would match you with if you are nervous. Pokey-Joe would be a lot more enjoyable. Enjoyment is the reason we ride.
Also consider If it's really making you nervous riding a different horse for a while or borrowing someone's who's really slow to help build your confidence up! Trust me it works! And then when you feel better get back on your horse and don't think about it. She doesn't want to scare you!
Thanks so much, I've had a kind of bad fall off her 2 years ago and have been nervous since then. Every time someone tells me to canter, I get nervous and tense up. I end up learning forward and since my horse is an ex racehorse, she takes that as a que to go faster and I fall off