What is despairing of Allah's mercy, what are its consequences, and how can it be removed from a Muslim's heart?

Despair of Allah's mercy is when one loses hope in it and feels that they will never be forgiven, and that they are inevitably headed for Allah's wrath and punishment, and that repentance will not benefit them. Allah has condemned despair in the Quran, and the Prophet, peace and blessings be upon him, warned against it. Ibn Mas'ud considered it one of the gravest major sins. Its remedy is for a person to live between fear and hope, so they neither despair of Allah's mercy and forgiveness nor rely on it to the extent of abandoning good deeds.