How can one regain the sense of life and pleasure after being exposed to doubts about Islam that lead to constant contemplation of them, and how can one realize if one is in danger?

Nothing is more beneficial for a person's well-being than contemplating the Qur'an, understanding its meanings, and continuously reciting it, alongside frequent remembrance of Allah (dhikr) and persistent supplication (du'a). In this lies the life of the heart and the expansion of the chest. One should also seek righteous and sincere companionship, and avoid places of doubt and bad company. Allah Almighty commanded this when He said: "And recite what has been revealed to you from the Book of your Lord. No one can alter His words, and you will not find any refuge other than Him. And keep yourself patiently with those who call upon their Lord morning and evening, seeking His countenance. And let not your eyes pass beyond them, desiring the adornments of the worldly life. And do not obey one whose heart We have made heedless of Our remembrance and who follows his desire and whose affair is ever [in] neglect." (Al-Kahf: 27-28).