What should a girl do who obeyed God in His commands, then committed the grave sin of fornication and repented and sought forgiveness from her Lord, but she cannot forgive herself and lives in intense torment and self-reproach, and fear of death because of this grave sin?

Do not despair of the mercy of Allah if you are sincere in your repentance. You must renew your repentance, frequently seek forgiveness, and perform righteous deeds. Stay away from bad company and everything that leads to what is unlawful, and keep to righteous companionship.