How can one achieve sincere repentance and soften the heart after a long period of disobedience and doubts, to the point where the Holy Quran and legitimate religious responses are accused of being from the whispers of jinn?

Sins have a great effect on corrupting the heart and leading it astray, in accordance with His, the Almighty's, saying: "No! Rather, what they used to earn has stained their hearts."

The cure for hardness of heart and falling into sins is to stay away from sins, turn towards the Book of Allah in recitation and contemplation, and increase supplication and turning to Allah, in accordance with His, the Almighty's, saying: "Say, 'Indeed, Allah leaves astray whom He wills and guides to Himself whoever turns back [to Him].'"

Be keen to associate with the righteous and sit with them, and stay away from bad companions and corrupt environments. For the repentant person, abandoning the places where he committed sins and cutting ties with the companions who aided him in them, and replacing them with the company of people of goodness and righteousness, strengthens his repentance.

Beware of despairing and losing hope in Allah's mercy, for it is one of the major sins. Allah is Forgiving and Merciful, and out of His love for forgiveness, if we had not sinned, Allah would have taken us away and brought people who would sin so that He could forgive them.