Golfingdad, I think you're totally missing it. I've had the very experience the OP is talking about - more than once. I've played with other golfers that went from hot to not. Afterwards, they admitted the thought of shooting their best score ever got in their heads and that was it. You don't need a tournament to feel pressure. Any weekend game will do.
Can golfers feel pressure when they're about to shoot their personal best? Sure. But that doesn't explain why players get hot on the back nine after playing poorly on the front nine, right? How nervous are you really on the first tee?
Iacas, i am speaking metaphorically, but you're way off with less than 9%. I'll take the wisdom and experience of athletes like Michael Jordan, Peyton Manning, John Elway, Mike Tyson, Muhammed Ali, George Forman and Wayne Gretzky and many other atheletes including Olympic gold medal winners that stress the high level of importance on their respective mental game. Further, golfers like Jack Nicklaus, Arnold Palmer, Gary Player, Tiger Woods and Phil Mickelson have all said golf is 90% mental. Teach what you want, but more accomplished experts on the subject agree golf is 90% mental.
I've probably over-estimated the 9%. Look at the PGA Tour. The vast majority of the players probably have nowhere near the mental capacity (golf-related, I mean) of Tiger, Jack, or Hogan… yet they win.
Seriously, consider how much 9% would be if it truly contributed to a golfer's score.
Plus, those are professional athletes. You simply can't compare them to an 8-handicapper or a 12-handicapper playing in a casual game.
And I'm sorry, but I'm not letting you off the hook like that. If golf was 90% mental, why does Tiger Woods spend so much time practicing? Why was Michael Jordan one of the hardest working basketball players? Why does Tiger Woods spend so much time working on his swing? And when he's not doing that, he's working out, or running, or training in other ways.
How much time do you think they spend working on their "mental game"? Surely if it was even 50% of what separated them they'd devote time to it, right?
The "90%" is stuff that professional athletes say. Again, if you could transplant Tiger's or Jack's or Ben's brain into the body - keeping their swing - of a sucky golfer, he's still not going to break 90. It's just not gonna happen.
P.S. Jack, Arnie, Gary, Tiger, and Phil are not more accomplished at teaching the game than I am. Not even close. And it's been pretty well established that what pros say they do is not all that important.
P.P.S. Just off the cuff, I asked @david_wedzik - who had Web.com Tour status, has played on the PGA Tour, etc. - what percentage he would ascribe to playing good golf. His answer? 5%.