Good enough?

I'd love to know who defines what is good enough?
I'd love to know where my own personal perception of "good enough" has come from and why, after 33 years, two bestselling books, two amazing children and a host of other achievements I still feel simply not good enough.
I'm not sure what exactly I don't feel good enough for - except that there are days when it just all feels a bit pants and I wonder what I'm trying to achieve and whether or not I will ever achieve it.