Looking at it from a positive perspective we can see how we are constantly changing as designers and developing new styles and trends, while learning from our mistakes. We can almost guarantee that we each own a piece of work that we designed years back, but today we would look at the piece and say "what were we thinking?"
The negative perspective of this feeling of never being satisfied is exactly that... we are never satisfied! A job is never fully complete because changes can always be made to make something appear better, convey the message more clearly, or simply make us happier! As designers, we must realize that there has to come a time when we have to step back and say this is it!
If you have ever experienced this feeling of never being fully satisfied do not worry, you are not alone! This is simply the designer blues and the only cure is to keep designin'!
So when designing remember this:
Challenge yourself. Take risks. Ask for other designers' opinions. Keep learning new things. And mostly, be yourself because the hardest judge on your work out there is YOURSELF!