What is the ruling on doubt regarding religion, the authenticity of the Qur’an and the Messenger, and the continuous thinking about Hell and death, despite seeking forgiveness and striving in worship?

Fear of death and torment is religiously مطلوب (desirable/required) if it motivates a person to increase good deeds and prepare for what comes after death. However, if it leads to despair and lethargy, then it is a waswas (whispering of Satan/obsessive thought). Disliking death is often due to attachment to this world, or a desire to increase good deeds, or a feeling of falling short. To treat attachment to this world, one must realize that it is fleeting and the Hereafter is the eternal abode. A Muslim must acquire piety and prepare for death. Bad thoughts regarding Tawhid (Oneness of God) and Aqidah (creed) are satanic whisperings, which can be overcome by seeking refuge in Allah, relying on Him, consistently performing dhikr (remembrance of Allah), reciting the Qur'an, observing obligatory prayers, and supererogatory prayers, and not succumbing to these thoughts. The occurrence of these thoughts and feeling distressed by them is a sign of faith, and Satan only comes to divert the believer from it. One must not pay attention to these waswas, and instead increase the remembrance of Allah, recite the Qur'an, and seek refuge from Satan. If the waswas persist, one can consult a trustworthy Muslim doctor.