Make sure she is listening to you on the ground and in the school to start off, and then as other people have suggested, not going at the same speeds in the same places, half halts, asking her to stop, and lots of praise. Best of luck!

 I wouldn't change her bit unless you absolutely have to. Instead I would  try to get her to listen to your seat. That way you way collect her walk right up just by squeezing with your knees a bit and holding her through your core muscles. this along with half halts via the rein should get her listening to you and working at your pace. Hope this helps a bit.