When I first started riding Stoeka, she was a speed freak and every now and then she would run away with me. I was taught that if a horse starts to gallop and won't stop, don't pull with both reins because she will just lean on them. Just pull one rein until her nose is just about touching her side. She will have to slow down because in order to go fast they need to be balanced, and when you turn their head they are unbalanced and have to slow down.
If it isn't an emergency, then you can try circles like meantioned above. When you want to slow down, stop moving with the horse, sit deep and relax. If they are cantering too fast or you want to slow down to a trot, instead of moving your hips forward with the horse, move them backwards against the horse's stride (if that makes sense). She should then get the point and slow down.
Also, always make sure you are not subconsiously kicking her while you are asking to slow down. I know you should know that, but just check, because it does sometimes happen without you knowing. LOL.