What should I do, as I have despaired of my life after repenting and suffering from intrusive thoughts regarding purity and prayer, and then a new intrusive thought came to me about being a disbeliever for reviling God, religion, or the Prophet, whether intentionally or unintentionally? Are my prayers and good deeds in vain?

The true problem for the questioner is obsessive-compulsive disorder (waswas al-qahri), which corrupts both religious and worldly affairs. Its cure lies in ignoring it and proceeding with acts of worship correctly. The devil's insinuation that there is no benefit in worship aims to make the servant abandon it, as Ibn Abbas said, "What can Satan do with a ruined heart?" It is obligatory to disregard these whispers in matters of faith and worship. Even if we assume polytheism (shirk) has occurred, the solution is repentance from words and deeds that displease Allah. This is the logical course of action, for disbelief is not an inherent characteristic of its perpetrator, and whoever repents is saved. One must abandon these whispers and turn to Allah with repentance and righteous deeds.