Why Performance Is Different from Practice
Many of my students come to their lessons prepared, but do not play very well. They tell me that their last run-through in the practice room went without a hitch, but as soon as they come in for their lesson, they get rattled somehow and never play at their best.
I can think of three possible reasons why a musician can play well by themself, but not in lessons or recitals:

It’s Thursday at 2:00, and it’s time for Jonathan’s lesson. He comes into my studio and sits down and chats with me while getting out his music. He tells me that he is excited to share the work he has done on one of his recital pieces. But when he plays it for me,
You’ve put in hundreds of hours of practice on your program, your dress rehearsal went really well, and this is the best preparation you’ve had for any performance in your life. Why, then, ten minutes before walking on stage, are your hands shaking uncontrollably? How are you going to make it through the whole program without succumbing to the nausea?
It is therefore important to remove as many obstacles to getting started on the daily work as you can.
Are you a procrastinator like I am? It’s not that I don’t think ahead or try to make practicing for my next gig a priority. In fact, the more I think about how important it is for me to get to work on learning the music, the less inclined I become to sit down and actually begin the job. I definitely see myself in Robert Benchley’s famous aphorism: “Anyone can do any amount of work provided it isn’t the work he is supposed to be doing at the moment.”
In my previous two articles, I provided you 
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“I’m sooo sorry, Mr. Bach.” I said this under my breath as I walked offstage from a catastrophic performance of the Prelude and Fugue in G Minor from Book II of the Well-Tempered Clavier. It was just the first piece on the program of my recital at my parents’ church when I was in college, and I had blown it. The prelude had fallen completely apart. The fugue had gone surprisingly well, if a little shaky, after the prelude meltdown. Right then I couldn’t let myself worry about what had caused everything to go horribly wrong, I just had to gather my thoughts and get back out there to play the rest of the challenging program.