It sounds to me like she suffered from mental illness, especially depression. David suffered great angst expressed in the Psalms. Do you really think Catholics are not Christians, some of their doctrines are very Biblical Christian teaching. I am trying to study the history of Christianity without bias. The Catholic church became corrupted by greed, power and politics. The same thing is happening to fundamentalist churches. I was raised southern baptist, thankfully, God opened my eyes, opened my parents eyes, to the selfishness, petty narrow mindedness, arrogance, ignorance, greed, political idiocy stubborn hard heads and cold hearts of baptists.
It is neither works or faith that saves, it is love. Mother Theresa had a lot of love. She tried to fight the darkness, and it is harder to fight the devil when you have a problem with mental illness, your brain, soul is wired differently than others. You have no idea, when your heart is broken, over and over again, by living in a world full of evil and darkness, you don't know if god or the devil is the enemy, like they play with human souls as if it's a game.