Well.. ride it out is what I think. If she goes fast on a loose rein at the trot, start posting and add some half halts in and regulate her speed with those two things. Take her into large figure eights and serpentine 3-4 times down the arena both ways. But just help her ride it out, since she needs to know even if the rein is loose, you are still in control. Also don't be afraid to ask for a walk on a loose rein, from your trot. When she complies, then ask for the trot again on a loose rein, adding in lots of half halts and making sure you aren't asking too strongly with your legs because she may just be going fast on that basis.
Also, don't just give her the entire loose rein, start with contact, and slowly give her a hair of rein more when she is where you want her. Eventually you'll be able to give her more rein and still maintain speed. Or keep her long and low on contact and maintain speed.. it takes lots of practice :)