How can a sinner who has repented of his sin, and who contacted a girl online, then discovered that her name does not exist in her country, and fears that he may have harmed her, or that she may have lied to him or bewitched him, deal with these whisperings and pangs of conscience?

Conversation between a man and a non-mahram woman is a gateway to evil. A Muslim should initiate sincere repentance from it, and not pay attention to pangs of conscience or fear of harming her; these obsessions may be from Satan to disrupt one's life and drag him towards forbidden steps. A Muslim should strive to follow the path of righteousness, seek refuge in Allah from Satan, and initiate marriage if he has not yet married.