What is to regret bitterly?

Regret bitterly means to feel sorry, remorseful or sad about something that has happened or been done in the past. It implies a strong sense of disappointment, guilt, or self-blame, often accompanied by a desire to change the past or make amends for the mistake or action that caused the regret. The term 'bitterly' emphasizes the intensity of the negative feelings associated with the regret.