With Mom’s life on the line, Bella experiences a moment of clarity. It would be unthinkable to not play along with James and leave Mom to die, so in Bella’s mind, the only option is to betray her new chosen family and sacrifice herself. Earlier in the novel Edward said that Bella would never be in the position of having to sacrifice herself to save someone she loved. But now she is, and she is willing to commit that sacrifice. That this sacrifice involves also giving up Edward’s love amplifies her selflessness. Finally, note that throughout the novel Bella has struggled to contain her emotions. Now when her plan depends on it, she does master them.