With an untrained horse you have to take it easy, but not too slow as to bore her. Just a good, progressive pace. Patience is the key. When trying to stop her, I'd make sure you have a soft feel of her mouth. You shouldn't have a lot of hold on a horse that's just been broken in. Relax your leg, knee included. Usually when I help with the pony babies we use voice commands, and those really help. So if you say "whoa" firmly and softly while you ask her to slow, she should eventually catch on. Since we don't really know much about your horse's mind and what she does with a rider on her, our suggestions probably are not as beneficial as they could be.