I read and copied it (by hand) without attribution many years ago. There's a sadness in my inability to attribute authorship to words powerful enough to write about some 15 or so years later. A significant learning in the intervening years is the wisdom of always noting the source, whether a book, a lecture, a website, a speech, or a private conversation. If it's important enough to remember, it's important enough to remember the source. Though I've tried, I've not yet been able to find the source (and, yes, help is welcome).
What I do remember is why I kept this quote for (re)discovery. It captured much of what I wanted to convey to my now-grown daughter about the difference between striving for perfection (by some external standard) and accepting who we are. Striving to achieve the unrealistic goal of Perfect derails the very efforts that lead to excellence, as well as the efforts that lead to failure--the failure required for learning.
This doesn't excuse mediocre work or cutting corners or accepting less than our best effort. What it does, however, is free us from the debilitating effects of chasing a perfection we will never reach. It frees us from the tyranny of perfection, the tyranny that most often blossoms as procrastination.
Knowing we cannot reach perfection, we fail to reach at all. Far better to start with an honest assessment and acceptance of who we are and what we know, then find the people and the opportunities to challenge us to personal excellence.