How can certainty and faith be restored after a period of commitment to acts of worship, if doubt has begun to creep into the heart regarding the existence of God and the truth of Islam, leading to a loss of the sense of the beauty of worship and fear of the future and death, while continuing to perform prayers as a routine?

You must cut off obsessive thoughts and not succumb to them, for Satan is keen on misleading believers. There is a difference between the whispers of Satan, which do not affect faith, and doubt, which harms it. Increase your supplications, adhere to righteous company, maintain the recitation of the Qur'an and remembrance of Allah, and attend congregational prayers. Do not believe that what you are experiencing is magic or possession, for with the help of Allah, the schemes of Satan can be repelled.