I have an object that uses many helper objects to send messages back and forth. For some reason, I struggled with the exact implementation of those helper objects and just couldn't get them quite right. My tests would pass, but I would get stuck on the code and didn't get an API fleshed out.
This morning, I realized that I had it backwards. I'm now writing the main object and the API of the helper objects is incredibly obvious. I don't have to stop and think about how they work, I just know. Work is going much faster now. I'm going to have to remember this in the future.