Only the regrettable can teach you something. It can show you where your narratives went wrong and where you are not taking up responsibility for your life and your pain.
The good side of regret guides us to a better understanding of ourselves and a place of acceptance and growth.

Our narratives are the way our minds construct events to explain our feelings and experiences. 

When we experience regret, we are choosing to replay our broken narrative over and over again. Our problem is that we take these failures on as our lost identity - the person we should have been. Then we torture ourselves with that idealized image.

Our narratives are chronically short-term, emotional, and self-centered.

Regret can be seen as a mistake that we haven't learned the proper lesson from yet. If we learn from it, that mistake becomes helpful and makes us better.