How do I deal with the fear of missing a fatal flaw?

You are haunted by the failure modes you might have overlooked, and the fear of a catastrophic error follows you around. That fear comes from taking the work seriously, from knowing that what you design can truly hurt people if it fails. It is not irrational. But left unchecked, it can run you ragged and cloud the very judgment it is trying to protect. Here is what to hold onto. You cannot guarantee perfection, and no engineer ever has. What you can do is work the problem with discipline, use your margins, run the checks, and let a sound process carry the weight your worry is trying to carry alone. The fear wants you to believe that only constant dread keeps disaster away. It does not. Rigorous method does. When the dread rises, turn it into one concrete action instead of a spiral. Pick the failure mode you are most afraid of and go verify it, once, properly. Then trust the result. Channeling the fear into a specific check, rather than endless what-ifs, is how you honor the responsibility without letting it consume you. Your margins are there for exactly this. Learn to lean on them.