How can one get rid of the obsessive whispers concerning hypocrisy, God's عدم مغفرة [lack of forgiveness], and torment for major sins, despite sincere repentance?

Usurious loans are grave major sins that necessitate sincere repentance with its conditions: remorse, abandoning the sin, and resolving not to return to it. If these conditions are met, then repentance is accepted, by Allah's will. Despairing of Allah's mercy is impermissible, for His mercy encompasses all things. The feeling of not being forgiven is a whisper from Satan, aimed at causing the repentant person to despair. One must engage in sincere repentance, and if possible, return the principal without interest, as that is what is required. Otherwise, repentance alone suffices.