And just like that... it was gone.
"At first I didn't want to think too hard about it. Sometimes when you look too closely you can't see things clearly anymore."
Apparently I looked too closely.
Relaxed focus ---> the zone ---> optimal performance ---> excellent results ---> self-reflection ---> tense focus ---> not the zone.
It doesn't seem that self-reflection should be a negative, and I can't say that it caused me to become self-conscious. The truth is that I don't know why last night's session turned bad. That's not so say that I played horrible or lost a bunch of money. But I did become frustrated to the point where I cut short my work for the evening.