I get nervous about cantering my horse, partly because he tanked off when I tried him out pre-purchase (although I know that was my own fault!) and partly because we'd a bad fitting saddle that was causing a problem and he used to sort of rear when I'd ask him to canter...
I do know from the pre-purchase tanking off that he can and will stop when asked, and that once he's cantering everything is fine, my problem is the transition into canter. I give rubbish aids (panicking) and start to flap, which doesn't help. So I pretend I'm teaching someone else how to ask the horse to canter, I ride in an arena, so I ask coming out of the short end and tell myself to do what I'd be telling someone else - sit back, hands down, inside leg on, outside leg back and can-ter!
It sounds so easy when you're saying it, but actually doing it, is something else!
The other day when I was riding I was telling myself to sit back, and the sunlight hit my chesticles, and I was temporarily blinded by the glare, which made me laugh, so (having missed that transition) next time I asked I was so busy giggling about blinding myself with the glare that I got my canter! If I'm feeling particularly nervous I just canter down the long side to the next corner, if I'm brave I'll go around the corner (horse likes to lean in, and my balance can be a bit "off"). Some days I even manage a full circuit!