You sound like stress/self doubts are your biggest issue. There's plenty of people working in tech who are objectively not very good, but they get along winging

You sound like stress/self doubts are your biggest issue. There's plenty of people working in tech who are objectively not very good, but they get along winging it and pretending. Obviously, that shouldn't be your goal, but remember it as your safety net. The secret of your high-performing people might simply be that they put in the work and spent 100+ hours reading the current codebase and, thus, they now know exactly where to change what. But they didn't start that way. They started out being unproductive and reading the source code base.