Jason Fried:

A better path is to reflect forward, not backwards. Develop a loose theory while working on what’s next. Appreciate there’s no certainty to be found, and put all your energy into doing better on an upcoming project. But how will you do better next time if you don’t know what went wrong last time? Nothing is guaranteed other than experience. You’ll simply have more time under the curve, and more moments under tension, to perform better moving forward. Internalize as you go, not as you went.