Take practice and discipline and work it into skill. Couple that with making the right decision at the right time, motivated by the desire to win and you have heart. Take that "heart" and let it get it overpowered by ego and showmanship and you'll have the formula to play poorly at times and throw a temper tantrum.
Humility goes a long way in the game of pool. Don't crow like the barnyard rooster when you beat an old man or some kid just learning the game.
I've been beaten in tournaments by guys that had no business beating me. They wing-dinged a few 9 balls and they went in. The cue ball launched into the air two feet and came down on the 9, sending it 8 feet to the pocket. I always smile, offer and handshake and tell them they played well. It shows younger players how to accept losing. If you can't be a gracious loser, the ego may keep you from ever being a good winner. If you won't respect yourself, nobody will.