I like this: "A person’s success in life is determined by having a high minimum, not a high maximum. If you can do something really well but there are other things at which you’re failing, the latter will hold you back. But if almost everything you do is up there, then you’ve got a good life."
I've heard it said (maybe by Frank Zappa? can't remember for sure) that the difference between a professional musician and an amateur is not how well they play at their best, but how well they play at their worst.
There will be many concerts, where you are just at 70% of your peak ability.
As a professional musician, you have to make sure your 70%-version is good enough that people gladly pay for seeing it.