Oh lord, I thought I was the only one! My horse loves going fast, once I ask him to trot, he goes all out. I can get him to slow down by slowing my rise when I post (like Clava said), or by talking to him in a calm voice, asking him to take it easy or slow down. It just takes practice to keep it under control. Try shortening your rein, too. You don't need to be in your horse's mouth, but just keep gentle control of her head. That way, when she gets too frisky, you can just apply a tiny amount of pressure and she will get it right away instead of being on a longer rein, pulling back, and getting in her mouth.
Best of luck!