What do you currently do to try and get her to slow down? I am wondering if you're maybe pulling on both reins? Is someone who is riding her? If so, you're creating something for her to brace against and GO. A snaffle bit is designed to be used one rein at a time.
My suggestion if you think everything is good with her and your training, is to remove the cantor from the ride. The cantor should not have anything to do with speed, it is just another gait. One of the most important things you can do is work in transitions. Don't let her go more than a few steps in the cantor, and then, pull her back down to a trot. Don't LET her take off. If she ever increases the speed without you asking, you need to make that decision uncomfortable for her by making her work in that moment.... As was mentioned, she is running away and controlling the ride.
This isn't to be taken a negative, she may be running away to a point you both like to be at.... running free. It sounds like you have fun when you do it. But, you need to see that that is her taking control of the ride. You can go there. Sure. Have fun. But she cant go there first. That's why, of course, you're making this post!
My suggestion if you think everything is good with her and your training, is to remove the cantor from the ride. The cantor should not have anything to do with speed, it is just another gait. One of the most important things you can do is work in transitions. Don't let her go more than a few steps in the cantor, and then, pull her back down to a trot. Don't LET her take off. If she ever increases the speed without you asking, you need to make that decision uncomfortable for her by making her work in that moment.... As was mentioned, she is running away and controlling the ride.
This isn't to be taken a negative, she may be running away to a point you both like to be at.... running free. It sounds like you have fun when you do it. But, you need to see that that is her taking control of the ride. You can go there. Sure. Have fun. But she cant go there first. That's why, of course, you're making this post!