I believe in second chances. But not in third. At least not if the person still refuses to change his or her negative attitude. Life is too short to let bitter people steal your energy year after year. I know it feels terrible to end a friendship, but people who bully are not real friends anyway, so there's really no loss.
Just wish the drama queens well, say goodbye and move on with your life and I promise you, you'll feel a lot better. Don't take crap from anyone, because you deserve only the best in life!