I find the following thought helpful when I'm in a valley:
Perfecting your Karate is like cutting the corners off a cube: for every large one you cut off, several smaller ones appear; and though to a less advanced student you may appear as a perfect sphere, you are often painfully aware of the many angles that still remain. -Elmar Schmeisser, Advanced Karate-Do
What I take from this is that as we improve, our flaws become more and more apparent to us. This does not mean that we've gotten worse--rather, we have sharpened our perception.