How can one deal with the whispers of doubt regarding the existence of Allah and His mercy, and return to prayer and remain steadfast in it?

The cure for obsessive thoughts is to ignore them and pay them no mind, and to be certain that they do no harm as long as they are mere whisperings to which the heart has not assented. To increase certainty in the existence of Allah, reflect on the dominion of the heavens and the earth and the manifestations of His mercy, for every atom bears witness to the existence of the Creator. Accompany the righteous, attend gatherings of knowledge and remembrance, and strive in supplication. Preserve the prayer, for it is the pillar of religion, and reflect on the punishment for abandoning it. Through striving and patience in obedience, you will attain the sweetness of prayer and closeness to Allah. A good life is attained through righteous deeds, and misery by turning away from Allah, as He Almighty said: "And whoever turns away from My remembrance - indeed, he will have a difficult life, and We will gather him on the Day of Resurrection blind." And He said: "Whoever does righteousness, whether male or female, while he is a believer - We will surely cause him to live a good life."