I think the "own worst enemy" thing is what separates upper-medium skill players (like me) from the really excelent tournament players.
There's one guy I play who doesn't know the rules quite as well as I do, doesn't spend nearly as much time reading and thinking about the game, and doesn't really care as much about the game, but when he plays his execution is often flawless, and I only win maybe 40% of my games against him.