Hello Mike,
answering from the point of view of a depressed person, You may love her deeply, and whant the best for her. My advice is to help yourself first, you can't fight for both, becawse as cruel as this may sound, you will be dragged to. I'm not saying to lave her. But that you can't feel guilty for her condition or your feelings. Be her friend.... sometimes that's all we want. Just listen to her... don't try to understand. She'll come to you, and no matter how stupid you think her conversations are, just listen. Leave her as a fiance if you whant but not as a friend, becawse the person you fell in love with is still there. She just needs help getting back.