What I did was ask my instructor if I could canter by myself. She'd have everyone else canter, then stop and wait in the middle and let me canter. This way, I could work on my issues with traffic only at the slower gaits, yet still get some practice in at the canter. As for dealing with the traffic, when I'd start to get overwhelmed, I would break down to walk or halt in the middle of the arena, so I could take a few breaths and re-center myself. Lessons are supposed to be fun and still be a learning experience! It defeats the purpose a little to get overwhelmed by the number of people in the arena.