If you don't want to canter on the lunge line canter on the rail. If you're afraid to do that, listen to my long boring story :) 4-5 years ago I went to a lesson barn that had these green horses and they put kids, me, on their back and told me to jump lines, canter with 5 other people in a tiny arena, and other very stupid stuff. I fell off almost every single lesson. That is why I stated doing dressage, I got so scared to jump that I decided to quit. Over time I was still scared, but one day I was cantering over a pole and I had fun! To me, if you are VERY uncomfortable doing something wait. Try doing it a couple times or ever so often but if you're horrified then wait. Ever since I started dressage, I don't canter anymore (the horse I jump is green so we don't canter then) then my trainer told me to canter and I've been scared. I think it's because I don't feel like I know what I'm doing, because I'm not used to it. Now that I feel more comftorable riding, I am trying to canter more Becasue I want to push myself. A try both, waiting and pushing. Good luck!