Not only on the golf course.................

Good players have the power to think while they are
competing.  Most golfers are not thinking, even when they
believe they are.  They are only worrying.
     Worrying is a misuse of your mind on the golf course. 
Whatever your obstacle, worry will only make it more
difficult.  Further, it is impossible to make a good golf
swing if your muscles are too tense.
     Rather than worrying, be mindful of the shot at hand
and go ahead and play it as if you are going to hit the
best shot of your life.  You really might do it.

-Harvey Penick,  The Game for a Lifetime