Why do I keep missing the same flaws in my own work?

You have gotten good at checking your own work. Typos, gaps, obvious mistakes — you catch those now. But something else still gets through, and you learn about it later, from someone else, at a worse time than if you had caught it yourself. This is not carelessness. You are inside the work. The same assumptions that produced the flaw are the assumptions you would use to hunt for it. Ask a blind man to check his own blind spot and he will report all clear. That is not failure. That is what a blind spot is. Working with others changes the problem from personal to procedural. You do not need to become a sharper judge of your own work. You need a structure where someone else checks the part you cannot check — built in before you need it, not called in after the damage is found.