Ask your trainer to school her, or a use a leverage bits. I loooove gag bits and elevators. If you have soft enough hands, the horse can really learn from them. Ride in a gag or an elevator for several weeks. Start at the walk, then trot, then canter. Do lots of transitions to teach your horse to round its back and lift herself up. If she drags you in front of the jump, halt in front of the jump. It will not teach her to be a stopper. Get your horse trotting forward and having a nice head set. Work on getting her mouth to soften. I do not like waterfords because either it works really well or it doesnt go well at all. My horse mouth gets really hard and she leans on a waterford, while others get light and soft with it. Ask your trainers questions and have them help you pick out a bit. Although draw reins make them put their head down, it gives you a lot of control. A pony I used to ride was very spooky. She would take off a lot. Sometimes I would ride her in a elevator, double twisted wire, and sometimes with a regular d ring and draw reins. They each gave me a lot of control and helped her a ton.
Agree with Sofia completely but I have used a Waterford mouthpiece on strong horses especially those who are bull-necked. As the mouthpiece is a bit harder than normal ones you don't want to use a gag or something more forceful to start with at least.
The Waterford is designed so that horse cannot get a hold of it and run. Obviously, if you are having issues in canter, look at yourself first. Do you tense up? Are you reins unnecessarily tight before commencing? If your horse isn't schooled well, she will only see pressure as a stop thing and pull through that as you're already gripping and she's confused. May be worth getting someone with some experience an instructor or someone in to help you & your horse. Adding extra gadgets won't help.
I agree with Sofia but I've HEARD that waterford bits help with this. Again I've only heard by mouth and never seen it with my eyes so I can't guarantee. I would do some more research before buying one anyways :)
Remember that correct neck position and contact with the bit come are consequences not only of rein action but leg. Get your horse moving, with lots of energy and impulsion (not necessarily speed), put your shoulders back, sit deep in the saddle and dose pressure in the bit, reason with the horse. I read this quote from Charlotte Dujardin that says "think of your hands being in front of the saddle and always pushing the horse to the bit, not pulling the horse back". With some horses, the more you fight them, the more they fight back, so make sure you're not using too much strength.