I went to a language school in UK for a bit. My grammar sucked tbh but I was somewhat good at speaking. There wasn't much grammar study. It was more like training each pattern again and again. Like you train "so that" pattern in 10 different settings for example. Then I came to the states and just started working. You swim or drown situation. I didn't wanna drown obviously so did everything. I memorized thousands of vocabularies, read hundreds of magazines and mangas, watched tons of sitcoms and animes, practiced differentiating and pronouncing vowels and consonants, limiting myself to only search in English, basically anything you can think of except using Japanese materials as I didn't find it helpful. It'll be a book if I go into the details but you got the idea.