Reflect, but don't dwell
If you're driving down the road using your rear view mirror to guide you forward, how far do you think you will go? Probably not very far!!! So why do you constantly look back at your past and think about how you wish you did something different?
Dwelling on the past isn't going to do you any good. Looking will help as long as you're doing it to learn from mistakes and reflect on how far you've come, but outside of that, if you aren't looking forward, you won't get to your destination!