your horse doesnt' know how to earn herself a release from the bit pressure. IN fact , she doesn't even realize such a thing is possible. And when she goes from zero to 60 in a lightening bolt, and you react by pulling back really hard (which most anyone WOULD do), then you have both missed the place where you could have got her to give to the bit, infintisimally, and then given her a reward release.
So, you have to set up situations where you can do the "pressure on bit, horse gives to bit, rider relaases and praises horse" sequence , over and over again.
YOu have to be really sensitive to her giving even the slightest to your pull, because if she isnt' really aware that "giving" is what you are asking, she might not do it , or will kind of try this and try that and then, almost by accident, give to the rein. YOu have to be really paying attention and catch that tiny little give she offers. If you aren't paying attention, she'll give, you'll not see it and not reward it, so shell try something else, real quiick, to get a release from the rein. OR, she'll shut down and go into "pulllllllllllllll" mode.
So in this retraining, you keep setting up the situation where you apply pressure (one rein only is best) and watch eagle eyed for the "give" on her part, and reward it. Soon it will go from her accidently hitting upon the correct response to your pressure, to her offering the correct resonse, time after time, as long as you promptly reward this.
Then you can ask for more and more give before actually rewarding, and then go to applying two reins (always one more than the other, NEVER exactly even) and then move into backing her up and more!