It sounds like she suffers from deep seated depression and anxiety. I've been there- thought not as bad (I hope). Basically it boils down to obsessive negative thoughts and anxiety.

I'm sorry your friend is so difficult but I imagine it's even more difficult to be her. That kind of perspective is like living in a dark cage. But she might also have some challenges that you can't be aware of. She might have a tough life as well. Has she ever spoken of being depressed or upset with her life?

It's not for you to 'fix' if she doesn't see it as a problem. I have friends who can be like this sometimes. I had to learn that Sympathy and Empathy are two different things. This is where compassion is the key. See if you can be mindful of your own needs while being present with her?