What is the ruling on one who doubted the truthfulness of the Qur’an then repented, but still feels constricted and uneasy, finds no joy in worship, and fears that he is falling short in applying knowledge? And what is the remedy for this condition?

Repentance expiates what precedes it. It is feared that what the questioner is feeling might be whisperings (waswas), and the cure for whisperings is complete disengagement from them. It is advised to continue seeking religious knowledge, and to read books of Tafsir (Qur'anic exegesis), books of encouragement and deterrence, biographies, and stories of the Pious Predecessors, and to act upon what is learned. For knowledge coupled with action is a cure for the heart from hardness and the whisperings of Satan. Abdullah ibn Wahb mentioned that seeking knowledge removed the whisperings of Satan from him. And Sheikh al-Islam Ibn Taymiyyah conveyed that acting upon knowledge strengthens it, while contradicting it weakens it.