You might have found a solution, but I thought I'd post anyways.

I know how you feel, whenever I ride a new horse thats big. But I've noticed something: unless the horse you'll be riding has something wrong with his brain, he's not going to roll over. If he had something wrong, you wouldn't be allowed to ride him. Once you get on, just relax and have fun. Don't worry about him rolling over. It's not gonna happen(unless the world flips and he has no choice ) Hoope it helps, PM me if you want to talk more after you ride =]

Hope you have fun, I know you will!