It never ceases to amaze me how much of golf is in the head. I used to do a little thing with a pal when we played socially we would add up our mental errors. Both decent golfers, and both of us would pride ourselves on solid course management. It wasnt scientific, but we would keep a mental tally of what poor judgment, mental errors cost us during the round. Maybe even a quarter shot for a small error. You get the drift. We would invariably end up with 3 or more shots from mental errors! It was amazing to us and caused me to refocus on the mental side of the game.
Played in an event last week that I really wanted to do well at (scratch 27 holes). Always a mistake to have that strong desire as I hacked first 3 holes for 6 over. From there no way I could win but a Top 10 would be good, so had incentive to grind it out. Things improve and stand on 8th tee, easiest hole on course, just 330yds, straight, elevated small green with rough in front, so its hit it 220 or so then wedge at pin. For some crazy reason rather than just knocking it down the fairway I decide I will aim a bit left and fade it onto fairway. Caught a protruding branch of tree on left, ball bounced away into crap, result double. Two+ shots down the drain for a bad mental error. I finished 8th, and was 2 shots off being 3rd, so that stupid mental error cost me a Top 3 finish.
Playing 5 hour round in humidity its hard to remain focused, but when one lapses, it can be very costly. So the moral is, engage brain! (I may have that tattood on the back of my hand, always wanted a tattoo and could never think what to get).