When she throughs her head up, do you try to pull it back down by lowering your hands? This is only natural, but it can put a lot of downward pressure on the bars of the horse's mouth. You might try if she throws her head up, you too raise your hands to maintain the straight line of elbow to bit and put enough tension on the bit that it says '" no, not there, that's not the release point". She will go higher, left, right and eventually put her head or just tip her nose down, YOU RELEASE PRONTO. Of she puts her head downish and doesn't walk forward, call that a day and go on forward again.
Kevin has a point, it was on my mind too, in that too much backing associated with each and every stop will irritate her and eventually, ruin her forward.
It's important that when there are circumstances that require you to put on a fair amount of contact (and it might take a lot, but it takes what it takes), that you also try to find places to let the horse have just as much release as you can trust them with. Like on the buckle if you can do it. If the know that the rein won't always be tight, they are more willing to not think of it as something that is hopeless and all they can do is mindlessly resist.