How can one restore his faith in God and Islam, and remove the hypocrisy that has afflicted him, while continuing in acts of worship and da'wah, yet not feeling the sincerity of faith?

To protect yourself from the whispers of Satan, turn to Allah in supplication to preserve your faith. Recite the Quran, and remember Allah morning and evening. Avoid sins, especially looking at nude images; repent from them with their conditions. Stay away from bad company and gatherings of suspicion. Address any psychological, social, or financial problems that might be a cause of the whispers. If what you are feeling is depression and anxiety, consult a psychiatrist.