What is the ruling on feeling regret for not committing a sin?

We ask Allah to keep you steadfast in repentance from this sin. Remember that the consequences of sins are grave, and they are a cause of alienation and distance from Allah. They may deprive you of your sustenance and lead you to Hellfire. Whenever you remember the sin, recall its consequences. This might be a trial from Allah or a whisper from Satan, so you must supplicate and seek refuge in Allah from Satan and his whispers, and engage in righteous companionship and good deeds.