How do I get better at catching the hidden flaws in my work?

You can look at your own work and judge the surface now. That is a real step, and most people never reach it. The flaws that still slip past are the subtle, internal ones, the kind that hide under a clean face. Those are the hardest to catch precisely because they do not announce themselves. The truth is, no one spots those by staring harder in the moment. They are learned through pattern, over time, by seeing many that failed and understanding why. Every flaw you miss now and later trace back is not a failure. It is one more entry in the library your eye is building. That library is what expert judgment actually is. So slow down on the finish, and study the ones that got past you. When something is found later, go back and look at it yourself. Ask what sign was there that you did not yet know to read. That habit, kept up, turns misses into skill. The subtle flaw is not beyond you. It is just further along a road you are already on.