At first, you should check if your horse doesn't have any pain of the left side of her body.
Then, when you're sure she is okay, make sure you give her a proper warm-up, very symmetrical.
If she wants to fidget a bit at the begining of the session, let her so she will shrug off her energy.
Then, when she is warm enough, do a lot of simple exercises to keep her focus. Ask her to be on the bit with her hindlegs engaged keeping the impulsion.
If you keep her busy enough, she will not have time to think about doing wrong.
When she behaves well, don't forget to give her breaks and a little caress so je understands that it is easier for her to be good :)