Most Helpful Guy
I don't think you can tell her anything to help her. As you said she just doesn't listen. The words simply don't register because she's in that negative mood. In order for her thoughts to change, the mood has to change first. Positive and wise thoughts don't live together with negative feelings. They don't get along at all.
Don't try to fight her negative thoughts with more positive words, it doesn't work. In order to help her, you have mimic the positivity and openness she can relate to. It's not going to be so much verbal thing, but energetic. If you say her "don't feel or think like that", that's not positive thing to say. Openly (meaning no expectations, no agenda) be with her and listen to her, but don't take the negativity too seriously.
But there is also big challenge for you. You can't get frustrated and angry at her if she doesn't listen to you or change. So don't be result oriented. The only important thing is to do your best to listen, understand and at the same time provoke positivity in her, but not verbally. Thoughts and beautiful words are not important now, but feelings and emotions. And you have to keep yourself in the positive side.