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.

Their code has proper error handling with custom exceptions, input validation, thread safety considerations, clean abstractions, and documentation. Mine works but feels fragile in comparison.

In a recent mock interview, I implemented a simple inventory system. It worked fine for the happy path, but I realized afterward 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.

The frustrating part is that I know about these concepts. I just don't instinctively apply them when writing code. It's like I have to consciously remind myself every time.

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?

I'm not looking for a list of topics to study. I already know I should learn about exception handling, concurrency, SOLID principles, etc. What I'm really asking is how to internalize these things so they become second nature.

Any advice appreciated.