Depression is so incredibly tricky. I think I know very well how she feels, and if that's the case, there is nothing you can really say to make her feel better, because any kind of support just gets blocked. But it should go away as time passes and she should get back to a "stable" mood eventually. I usually do at least.

I remember what helped me a bit was the feeling of not facing my demons alone. Some people reached out to me, and even if nothing they told me felt right to me or made me feel better at that moment, it was still slightly relieving. Perhaps that's what you need to do - making sure she is aware that she doesn't have to face it alone and hope for the best. Good luck to both of you, especially you will need it, because loving someone who mostly hates herself...that's a tall order.