I'm working through a similar issue. I was leasing a horse with a really "big" canter (he's still a little green, and I'm a lot green), and the trainer I was with would do the scream routine and I'd get all nervous and the canter would basically be me desperately trying to balance. Since I wasn't really riding the canter so much as holding on for dear life, I wasn't controlling the horse and he would take it as an opportunity to run like a deranged dreidel around the arena. Eventually I had my other trainer come out and she pointed out the fact that I threw all my aids away when I went into the canter and they came back when I dropped to trot, which I hadn't realized on my own. After that I focused really intently on riding at the canter, and things got better so fast it was funny. You might be much more advanced, so please don't take this the wrong way, but if you're anything like me, I'd suggest just forcing yourself to do it a few times and really exaggerating your cues. For me, it made the horse listen, and then I felt a lot more in control, which relaxed me tons.
"...and may your life be filled with good horses." — Buck Brannaman