St. Thérèse understood temptations to suicide.

There is a sort of pious myth that pretends that devout souls never experience the “dark temptations”. In the imaginations of some, it is almost unthinkable that even the thought of temptations to grave sin could enter into the minds of the saintly. St. Thérèse’s life disproves this myth, and shows that is precisely the devout souls who enter into pitched combat with Satan to vanquish him.
Despite the profound joy for which St. Thérèse was known, during the intense sufferings of her last illness, temptations to despair and suicide passed before her mind. The agonizing sufferings that came from the slow death of tuberculosis and the dark night of soul that she experienced were enough to try even the strongest of souls. Her faith was her foundation and strength during these temptations, but she did share some inkling of the intensity of her pain with her spiritual family. Just a week before her death, one of her fellow Carmelites remarked on St. Thérèse’s sufferings, to which she responded, “Yes! What a grace it is to have faith! If I had not any faith, I would have committed suicide without an instant’s hesitation.”
A month before her death, she made an urgent plea to her Mother Superior: “Watch carefully, Mother, when you will have patients a prey to violent pains; don’t leave near them any medicines that are poisonous. I assure you, it needs only a second when one suffers intensely to lose one’s reason. Then one would easily poison oneself.”
One of the nurses who assisted St. Thérèse during her last days gave the following testimony during the beatification process:
“Three days before she died, I saw her in such pain that I was heartbroken. When I drew near to her bed, she tried to smile, and, in a strangled sort of voice, she said: ‘If I didn’t have faith, I could never bear such suffering. I am surprised that there aren’t more suicides among atheists.’”
That St. Thérèse spoke in such a way allows a small understanding as to the incredible pain she suffered. But, even more, it is a consolation and encouragement to Christians who are suffering under grave physical or psychological pain and who also have had the thought of suicide or despair pass unbidden before their minds. Here is a saint who understands, who has suffered the same, and who has claimed the victory. The faith was her rock, and Satan could not wrest her from that anchor.