Can one know whether a sin has been forgiven or not? And are seeking forgiveness and remorse alone sufficient for a sin to be forgiven after it has been repeated?

The believer combines hope with fear. He hopes for Allah's forgiveness, thinks well of Him, while thinking ill of himself. Thus, he strives in obedience, perfects his repentance, and fears his sins and that his repentance might not have fulfilled its conditions. It is not possible to be certain about the forgiveness of a specific sin until the Day of Judgment. However, if the conditions of repentance are sound and its pillars are fulfilled, then its acceptance is definite.