Can a Muslim suffer from mental illnesses, despite what some say about him not being afflicted by them?

A person may suffer from psychological illnesses such as sorrow over the past and anxiety about the future, which have a more severe effect on the body than physical ailments. These worries expiate sins, and enduring them earns the servant a reward. The treatment of these illnesses through lawful (Shariah-compliant) means is more successful than treatment with physical medications.

Among the lawful remedies: 1. Supplication (Du'a) with authentic prayers, such as the supplication of Ibn Mas'ud, may Allah be pleased with him: "O Allah, I am Your servant, son of Your male servant, son of Your female servant...", and the saying: "There is no god but You; glory be to You! Indeed, I have been of the wrongdoers." 2. Ruqyah (exorcism/spiritual healing): by performing Ruqyah on oneself, or going to someone whose religiosity is trusted to perform Ruqyah. For more details, one can refer to the books of scholars on the chapter of remembrances (Adhkar).