Wow, tough day. I don't see how any of that is your fault. Your BO, who obviously is missing the finer points of desensitizing a horse, took the reins literally and broke the bridle, not you. Don't know how you're supposed to be psychic and know she had already been exposed to bags. As far as you knew, it was a green horse, and not much more. I love it when horse people offer important information about their horse AFTER something has gone wrong. Feel better.
You just have to see your distance...you don't have to like it.