The difference is probably in time between shots.  In a tournament it's easy to maintain focus when your shots are fairly close together.  In league, you usually have to wait 5 minutes or so between shots and this can throw you off.  Taking a few extra seconds before your shot to refocus yourself on the task at hand is a good start.  Take a deep breath, cose your eyes and envision the ball arcing to the pocket or taking out the spare.  Remember, there's no rush, so taking your time is ok.  Once you are on the approach, set yourself and go.  Taking too long once the ball is in your hand and your feet are in place is usually detrimental to your game.
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Nut

And there were in the same country shepherds abiding in the field, keeping watch over their flock by night.  And, lo, the angel of the Lord came upon them, and the glory of the Lord shone round about them: and they were sore afraid.  And the angel said unto them, Fear not: for, behold, I bring you good tidings of great joy, which shall be to all people.  For unto you is born this day in the city of David a Saviour, which is Christ the Lord.  And this shall be a sign unto you; Ye shall find the babe wrapped in swaddling clothes, lying in a manger.  And suddenly there was with the angel a multitude of the heavenly host praising God, and saying, Glory to God in the highest, and on earth peace, good will toward men.

Luke 2:8-14