Taking it to the course

Why do so many of us have trouble with this?

Yesterday I was warming up on the range prior to my round and found that I was just striping the ball. Every swing was excellent, and it wasn't because I found a groove with a single club. Even as I moved from club to club the swing was there.

The positive thoughts carried to the course - through the 1st nine at least where I was just two over par. Then came the back nine where I lost confidence in my swing and struggled to bring it home at nine over par.

So what is it? I know what to do. I know to swing in a relaxed manner. But doing it consistently continues to evade me.

I think I might care about the outcome too much.