If you think about negatives situations, then that's what you get, a negative mood. That's not good for your health, for you or your girlfriend. If she is sad, you are sad because you care about her. Try to make things better by helping each other with any problems, talk about it...don't keep it inside. It is much easier to live happy than to live sad. Take a couple of deep breaths and let it out slowly...this helps to lower your blood pressure and releases stress. Go for a walk, listen to music, eat something you really like, try some fresh fruit, go see a funny movie, think positive. Just take it easy and be cool! Live, Laugh, Love.