What is the deterrent for pessimistic thoughts related to divine decree (qadar), such as apostasy from Islam, suicide, or straying after guidance, which affect one's mood and are feared to turn into obsessive-compulsive disorder?

The questioner is advised regarding two matters: First, firm belief in divine decree and destiny (Al-Qada’ wal-Qadar). Everything a person does is by the will and desire of Allah. The servant has choice and will to distinguish between what is beneficial and what is harmful, and he is held accountable for his choice. Second, submission to Allah's judgment and law, and not to overthink destiny. Rather, one should occupy oneself with beneficial religious knowledge, righteous deeds, and supplication. Destiny does not prevent work; rather, it necessitates diligence and effort, as the Prophet, peace and blessings be upon him, said: "Work, for everyone is facilitated." Ibn Al-Qayyim clarified that the pre-ordained destiny assists in deeds and necessitates them, and is not contrary to them. Imam Al-Tahawi mentioned that Islam is established upon submission and surrender to Allah, and that the essence of destiny is Allah's secret in His creation, and delving deeply into it is a means to failure. One should praise Allah for faith, ask for steadfastness, repel obsessive thoughts, and occupy oneself with beneficial knowledge and righteous deeds.