To play your best you need to be thinking of nothingbut playing.
Fixed it for you.
...but seriously, a lot of musicians and teachers and mentors etc. will tell you that "thinking of nothing but playing" is just a waypoint along the path to "thinking of nothing" ...or simply, "not thinking". The goal (ideally, in the Platonic sense) is to be so immersed in the music that all of the mechanics of executing on your instrument are completely subconscious, and your mind is freed to attend to the more spiritual task of being a vessel for the music to flow through [sic]
...and I won't deny that ^^^that sounds like a load of New Age fru-fru baloney. But it's a surprisingly common theme in musicians of the highest caliber. Sometimes you need to get out of the way of yourself in order to be connected to the music. Thinking About Nothing But The Music is a step towards that goal that comes before Not Thinking About The Music...or Not Thinking, period.