Does psychotic depression indicate weakness of faith, or is it a disease of the age and an illusion? Is there a way to realize that she is on the right path that pleases God before it is too late, given her fear of death and her excessive sins?

Illness is a tribulation from Allah, not an indication of weak faith. One must receive tribulations with patience and submission, for those who are patient will receive a great reward, and their sins will be expiated. One must seek treatment, occupy oneself with what is beneficial in matters of religion and worldly affairs, resist illness with strong will, and choose righteous companions. The path of righteousness is adhered to by performing obligatory duties, increasing supererogatory acts, seeking religious knowledge, and consistently engaging in remembrance of Allah and supplication.