One of my favorite quotes is from a book by Dan McCaw. 'There are no such things as failed paintings, only failure to paint.'
This quote could not ring with more truth. There are days when we work tirelessly on a painting only to finally call it done and only be halfway satisfied, and we then find it hard to find something good about that painting, but there is something good. You painted, and I guarantee you learned something, even if it was from mistakes. I find that most often I learn the most from the paintings that I feel I failed. I learned what not to do, and just hope that I do not repeat it.
Remember it takes probably 10 paintings to finally get a good one you want to sell, so keep on painting!