Immersion and speaking practice are important. Reading, textbooks, and grammar are important too, but that is just step one. Once the sentence structure has become second nature, immersion and being surrounded by the language, and speaking it even if you make mistakes becomes the top priority. Speaking even if it's full of mistakes is like sharpening a blade. If you don't speak or practice, over time your skills will decrease and dull. The first thing you forget is vocabulary. (This is happening to hakos baelz), then if you don't practice for a long enough time, you will forget grammar too, and then the language altogether. This happens to children who move abroad and adopt a new language and then forget their native language. I wonder if some japanese learners are too afraid to make mistakes, because it is embarrassing, or they might get bullied by their classmates. Is this the case?
………日本語は世界でも結構難しい言語らしい。仕事も忙しいだろうからあまり無理しないでね。 Apparently Japanese is one of the difficult languages. I'm sure you're busy with work, so please don't push yourself too hard.