How can an oppressor who does not repent be forgiven, and is it permissible to make forgiveness contingent upon the will of Allah Almighty? And what is the reward for a person with mental illness due to these problems?

Forgiving the oppressor means waiving the servant's rights, not permitting the unlawful. Forgiveness is recommended for the servant's right if it is sincere, and forbidden for God's right. The wronged person has three choices: forgiveness to attain the reward of the pious, or not to forgive, or retaliation without transgression. Forgiveness and pardon are best, and this applies even to an oppressor who does not admit his wrong, seeking God's reward. Affliction expiates sins and elevates ranks. One should combine psychological and faith-based treatment, have a good opinion of God, supplicate frequently, and stay away from sins.