What is the knowledge that yields regret, and which is the root of repentance according to Al-Ghazali, Ibn Hajar, and Al-Qari? And what is the meaning of Al-Ghazali's statement that the heart's suffering due to the loss of a beloved through one's own action is called regret?

The knowledge that yields remorse is the knowledge of the ugliness of sin and its harm to the servant in this world and the hereafter, and the knowledge of Allah's power and His awareness of the servant, and that His wrath cannot be withstood by the heavens and the earth. Remorse is the heart's pain due to its feeling of missing out on what it loves, and its regret over the action that caused this loss. Sins deprive the servant of the delight of closeness to Allah, His success, and His help. The loss of these great benefits due to sins yields remorse.