Well, mastery comes with practice. You can read plenty, that always helps, but it doesn't stick until you Do Hard Things in the language.
Or to put that another way...you'll learn to build better websites by first building a whole lot of terrible websites. Mistakes are how we learn. If you don't blush slightly at code you wrote two years prior, you're not growing.
Mastery is also relative. In one sense, I can be considered to have "mastered" C++ and Python, but I will always have more to learn. It's taken me six years to get this far in C++, and nine in Python.
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