I would agree that jerking IS NOT the answer. Clearly her past owner with the "not so quiet hands" has taught her to fear being slown down, or that slowing down is going to come with pain and pressure. It could be any number of things - bad teeth, an abcess in the mouth, bruising on the jaw, an ill fitting bridle/bad bit, fear of past experiences, etc. Rule out a physical problem FIRST. This may solve your problem. If not, then go back to the basics. Ask an experienced trainer/horse person what sort of tools are available for you to use that are gentle on your horse's face. I would almost retrain how you ask her to slow down - more with feet, seat and voice, leaving hands completely out of the process. (Eventually, that is!) Ask a trainer or coach to help with this - it makes for nice, gentle stops in the show ring and at home!