I think you'd be better off with a professional trainer, at least at first, because it's possible that the first stages of restarting won't be pretty. It will take someone who will not back down; someone able to meet the violence offered without emotion; someone who can, in fact, show he is stronger than your youngster, not so much that he has to "boss" her but that he is control of the situation. In other words, he has to be enough of a trainer to become not only the herd but the leader of the herd; at least when he's there. And I sadly admit I believe that there are times when fear comes before trust. Of course, so much depends on your horse.
You can probably get along without a pro, but it will probably take years, instead of weeks, to reeducate her view of the world; and in the meantime it will be dangerous.