"Being the boss mare is more than being a bully."
Attitude is everything. You are your horse's leader. You stand your horse on the side of steep, scary creek and say, "Walk on. Time to cross," but don't expect that horse to actually cross, that scared animal isn't going to go. You're the horse's leader; if you think it's okay and your energy tells that horse that there is nothing wrong and he is fully expected to cross without problems, he will cross.
I've found that having that attitude and expecting certain things out of your horse makes 'scary' situations a lot easier. If you aren't afraid, your horse will calm down as well.
You have to act the part of leader fully to be fully accepted as a leader.