I used to play a lot of poker online back in the day, two lessons stuck with me: Speaking to the variance of the game: "Eventually you will run worse than you ever thought possible" Poker teaches, contrary to most business literature, to not be results oriented. It is the decision not the result that matters. "You can do everything right and still lose".
This one. I'm not sure how many readers I would have if I tried to explain it in a book, but I'm tired (to the point that I don't read the books, just the covers) about how everything will be ok if you just wish.
When you just learn that sometimes people fail even having taking the best possible decision at any point, you get another perspective for your life.