"The longer you do something the less mistakes you'll make."
I think that's an amateur concept.
It may be slightly true, but that's not what makes you a professional. In fact, the more glitches in your performance, the more times you can prove to me that you are (or aren't) a professional.
The true measure of your professionalism is in how you handle (inevitable) glitches.
No one came to see you perform flawlessly. If they want perfect they'll pop in a CD or draw a circle with a protractor. They came to experience a real and meaningful expression, and if you're on stage they came in part to experience you. Not your ideal image of yourself, but the real you - glitches and all.
Your ability to handle glitches well - to move past them without allowing insecurity with your identity as a performer or an artist to take over, or to even allow them and embrace them as "character" in your art or as comic relief or as a way to create a common ground between you and your audience... that's what makes the difference between a professional and an amateur.