How can a woman overcome the state of depression and feelings of humiliation after an unlawful relationship with an atheist man who abused and exploited her, and can she seek forgiveness and repentance after that?

Understanding a mistake is the beginning of rectifying it, and Allah is Forgiving; He accepts the repentance of the remorseful, no matter how great their sin. Allah the Almighty says: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'" Have a good opinion of Allah, for He is as His servant expects Him to be, and He has promised forgiveness to whoever supplicates Him, hopes in Him, and seeks His forgiveness. Beware of thoughts of despair and hopelessness from Allah's mercy, for they are more dangerous than the sin itself. Turn to Allah with supplication that He protects you from the evil of those you fear, and frequently ask Allah for well-being. Conceal yourself with Allah's concealment, do not expose yourself, and begin a new page with your Lord. Be keen on the company of righteous women and gatherings of goodness. Marriage to a righteous man may help you. Beware of thoughts of suicide, for it is a greater misery.