Piano Lessons

Today I had another piano lesson with the cellist I'm playing my Christmas piece with. I was really nervous, and I messed up a lot. My teacher kept telling me to let go and improvise, but I'm finding that hard because I'm such a perfectionist when it comes to my piano pieces. I wish that I could let go and just play, but I think part of the reason is that I can't site read very well. Once I learn a piece, I memorize it and don't really look at the notes much after that, and since I can't recognize all of them at a glance, it is causing problems. I think I need to work on practicing my sight reading and really work on my theory. I also need to practice letting go of my perfectionism and just play and go with the flow when I'm playing by myself at home.