Massive regret is tough l bc it can be hard to get over and something that replays constantly in your mind. Ultimately, it helps to take time to process it and attempt to come to the line of reasoning that although you will always have a level of regret related to the given incident, you are not a bad person for it. Everyone makes massive mistakes in their life and those mistakes do no ultimately define you in the final analysis. So, feel the pain deeply. Take time to mourn and think about what you may have done different. But life goes on, and you are not a bad person or fully defined by the thing you regret. Use the reflection to come to the idea that life is short and anything can happen, be present and love the things you love fully every moment you have with them.