Simply put: 1/3 of your performances will be average, 1/3 will be great, and 1/3 will suck. Keeping this rule in mind can help add needed perspective throughout your career.
When things go wrong, it is just getting one of the sucks out of the way so it is more likely the next time will be a normal or a great.
As you continue to develop your skills and improve your standards change and greats become normals. Just keep in mind that as soon as we perform a great, oftentimes we expect it to become a normal. It is not a normal until you are reliably performing it 1/3 of the time.
Understanding the Rule of 1/3s helps keep you on the Mastery path.