What is the way to repel the strong whispers (waswas) and doubts that have afflicted me in my religion, and how can I ensure God Almighty's forgiveness for me, after having seen atheist websites claiming to be "ex-Muslims"?

The error in entering these sites stems from shallowness of knowledge. Islamic law has forbidden approaching places of doubt and atheism, due to the Prophet, peace and blessings be upon him, saying: "Whoever hears about the Dajjal, let him keep far from him." To correct this, follow five steps: avoid those sites, present doubts to scholars, earnestly supplicate to Allah to firm your heart, frequently recite the Qur'an and its exegesis, and read books by scholars to increase certainty in Allah and the truthfulness of the religion.