"Where is my faith?" she writes. "Even deep down … there is nothing but emptiness and darkness. ... If there be God — please forgive me."
A direct quote from her writings
She was experiencing something called THE DARK NIGHT OF THE SOUL. A simplistic definition can be found in Wikipedia:
Dark Night of the Soul is a term used to describe a specific phase in a person's spiritual life. It is used as a metaphor to describe the experience of loneliness and desolation that can occur during spiritual growth.
The "dark night" could generally be described as a letting go of our ego's hold on the psyche, making room for change that can bring about a complete transformation of a person's way of defining his/her self and their relationship to God. The interim period can be frightening, hence the perceived "darkness". In the Christian tradition, during the "dark night" one who has developed a strong prayer life and consistent devotion to God suddenly finds traditional prayer extremely difficult and unrewarding for an extended period of time. The individual may feel as though God has suddenly abandoned them, or that their prayer life has collapsed.
Rather than being a negative event, the dark night is believed by mystics and others to be a blessing in disguise where the individual extends from a state of contemplative prayer to an inability to pray. Particularly in Christianity, it is seen as a severe test of one's faith.