you can work with her in the ground on the lead, walk/jog with her and work on whoa and backing, trot from a halt or directly from backing etc. this gets her listening to your body language and basic cues--the main thing is getting her to listen and respond to your energy and the movement of your body. work up till you aren't using pressure on the leadrope to turn her or stop her(if you want, in an enclosed area tie the lead rope around her neck so you can let go and have her follow you around but yet still grab if needed). If you've got good rapor with a horse it usually doesn't take long to have them focusing on you and mirroring you. again the point is they are paying attention to you and your body language, rather than pulling their head around, is what gives direction as to speed and line. this is really productive before getting in the saddle.
What I have found that works great for slowing, is either circling, or some horses respond well to this thing I call "sit like mountain"
Circling, you use the biggest-smallest circle that works, start big and go gradually smaller till you find out where the horse will start slowing. Use all your aids to circle, and make them a bit more exaggerated for a more inexperienced horse. When you find that circle size, use that size. Horse rushes, circle em till they slow, when they slow let them go straight. Sometimes you'll have to bring the nose around to your toe and do a really tight circle. Try to use the LEAST amount of pull on the rein and really use your legs and weight to get them in that tight circle. Everyone needs to know how to do a one rein emergency stop anyways, so this is good to get the horse and yourself familiar with it--you with how much influence to apply and the horse in feeling it happen and keeping balance etc as it responds. Also, this circling technique is great to know when a horse shys or bolts, you can defuse the energy by letting the horse move its feet but not take off and runaway.
BUT learning the one rein stop technique have a professional SHOW YOU HOW first!!!
I just worked with a horse that rushed and kept trying to take off, I circled her tight, just once(what was needed to break the take off thought) and moved on. I also did a lot of backing(which she was good at) which helped a lot too--"firm" backing, stepping lively, like 8 steps then immediate move forward.
Make sure you are circling both sides.
I worked with another horse that responded well to the "sit like mountain" idea. I would exhale deeply while sinking all my being down into the ground, with as big a sense of relaxation and peace as I could be. He would immediately slow down for 5-6 steps at first, then speed up, by the end of the session(30 min btw) he was relaxed most of the time with a few speeds up when he saw something suspicious(it was a really windy day, so I was really impressed with him, he is a mustang)
dont' be afraid to experiement with what your horse will listen to and respond to. a slower gait should be a RELAXED gait, not just some robot slow pace that is lifeless. I think having RELAXATION as the goal, rather than a certain speed so much, is a better way to think about it, when relaxed the horse will naturally slow down.By far most horses rush because they are nervous, apprehensive, unsure, not listening, bullish etc. address the root cause instead of the symptom.