After a traumatic event, like a car accident, your mind spins with "what if" questions. The scenario goes on repeat in your head as you play every possible thing you could have done better, but didn't.
But asking what if I had done... is much less valuable than, what should I do next time? If something was out of your control, and you had no way to prepare for it, don't torture yourself with continual playback of the memory. If there something you need to learn from the event, take heed. Truly do something different next time.