A few things you can do.
One, you can just let her cruise along at a fast speed--either a high lope, or a long trot, or whatever she is she's doing. Allow her (or if you have to, make her) continue at that tempo until she starts getting tired. Then ask her to slow down. Then speed up. Then slow down. She'll start listening to you, because she wants to stop now, as she's worn out.
Or, you could put her in something bitless so that you can pull on her without hurting her. Ask her to slow down, and pull/bump until you get a response. Give and take, pressure and release. This will take many tries, but eventually she'll figure out that it's easier to listen to you. Make sure, throughout all of this, that you are staying very relaxed and not inadvertently cuing her to speed up.
A supplemental strategy is to take her over rough terrain, ground poles, hills, etc, while also doing serpentines and circles and so on, both to make her focus on you and to concentrate more on where she's putting her feet than on how fast she can go.