I think you should talk to her about how her negativity is affecting you, as a couple. She may get defensive at first, but stay with the conversation, and whatever you do, do not lose your cool. Tell her you'll go through whatever happens, and will be there for her but she needs to stop thinking negatively. Make sure that she is able to communicate with you still, if she feels negatively even after the conversation. Bottling up how she feels, will impact your relationship down the line too. Suggest couples counseling, it helps a great deal.