How do I transition from "code that works" to "production-ready code"?

I'm a backend developer with about 3 years of experience. I can solve problems and write code that works, but when I look at code written by senior engineers, there's a clear gap. Mine works but feels fragile in comparison.

In a recent interview, I implemented a simple inventory system. It worked fine for the happy path, but I realized after some chatting with ChatGPT that I hadn't considered concurrent access, didn't validate inputs, returned mixed types from methods, and used raw dictionaries instead of proper data structures.

For those who've made this transition:

• How did you develop the instinct to think about edge cases, error handling, and API design automatically?
• Were there specific resources, projects, or experiences that accelerated your growth?
• How long did it take before writing "senior-level" code became natural?

What I'm really asking is how to internalize the software engineering mindset so it becomes second nature.

Any advice or resources appreciated.