Have you tried to gently close your inside leg and asking her to bend? Sometimes that's all it will take for the horse to remember that you're still up there giving directions.
Since she is speeding up, I would sit a little deeper and ask for a half halt as well.
As far as why only at the canter... It's possible that she is a little unbalanced or uncomfortable in the canter, and this is her "excuse" (or opportunity) to let you know.
When you are approaching the corner what goes through your mind? Is it different, now, than the other corners? Sometimes the mere fact that we are anticipating something happening creates that exact response from the horse.