There is one question that i want to know from you?Do you still love her inspite of her attitude?If so, then better to sit and talk heart to heart what was the matter as she become like that. You know what, in a relationship, we are the one whose very emotional that we did not notice that we are doing such. Maybe its not the solution to break with her. Just show to her that you are faithful, and she's nothing to worry and eventhough its hard to show just always make her feel that you love her.