I've learned that being intentional is more important than being right.
Designing for real people will always be unpredictable. People are fickle, we get confused, we hang on to bad habits, and we out grow even the most amazing design solutions. I've realized that as long as humans continue to be human, I will continue to find myself in situations where the design I thought was "just right", is now totally wrong.
The best I can do is stop trying to be right, but instead wake up everyday and do the harder work of being intentional. It's important to be rigorous and deliberate about the course of action that leads us to our final solution - to understand why and have the persistence to grow and change alongside the people we design for.