Hi, Kitten Val,
I posted this in another thread, but I think it would help you as well:
have a few exercises to deal with a rushy horse:
1) set up trot poles - this makes your horse think about her feet and therefore slow down a bit. Make sure that they are spaced right for your horse's stride; not too close together, but not too far apart at all... if it feels like you're posting "big" and your horse is "skipping" to make the next pole, then they're probably too long; if you feel like you're on a jackhammer, they're too close. A variation of this (to work on collection, not being slow!! is to raise one end of each pole so it looks like spaced out crossrails - this will make your horse's back work as well.
2) Circle spirals - start at a 20m circle, trotting of course, and spiral (slowly) down to a 5-10m circle, whatever your horse can handle, then leg yield back out to the bigger circle, and repeat.
3) Circle whenever she gets rushy, and keep circling until she comes back down to the speed that you want.
4) Work on your own tempo - do a posting trot, and sing a little song (even your ABCs) to get a tempo for yourself, then try to get your horse to match your body. When your horse is truely balanced, they will have a perfectly 1-2-1-2 trot without any rein contact... which brings me to my next point:
5) Work on balance. You want to work on your flatwork (dressage!! hehe) and bending, and working into your hands and getting proper carraige of the horse's body. If you have a trainer, they might have different exercises to work on balance.
6) Transitions - do lots of walk-trot-walk transitions, take her (his?) mind off of just plain trotting.
7) Don't stress him/her out.. horses can get stressed and rushy when they don't understand things, so take it slow and easy don't forget to breathe... remember, horses get antsy when you get nervous, so make sure you're cool as a cucumber