My horse started doing that to me too. She would pull her head down at the canter and go faster, basically running away with me a bit. She would yank down and then I would lose the leverage that I needed to be able to sit up and pull to halt her. I know a lot of people, like another poster said, don't like aids, but I feel that sometimes they can be necessary (even for just a little bit), to help the rider correct a problem. So, I tried her out in a corkscrew bit and have not had any problems since. In fact, she is softer and lighter and we have never meshed better. I have gone back to the d-ring snaffle and she does fine and if she starts to get heavy again I switch the bit back. If your horse is not respecting your cues to stop or slow down, you need to change something and gain that respect back. Good luck.