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I can honestly say that my English is quite good. I watch South Park in its original language, play English versions of games, read English-language forums and websites, and have even read entire books a couple of times. I can easily communicate with foreigners on Skype, and, my personal favorite achievement, I have a good grasp of American slang. Well, its equivalent, anyway. How many subtypes are there? Several, one of them being gangster slang.
When I was in 5th grade and we were learning English, I was almost completely clueless. I didn't understand the teacher; maybe he wasn't explaining things well. I couldn't string a single sentence together. I alternated between getting 2s and 3s, with the occasional 5 for vocabulary dictations, which were the only thing that saved me.
But then, one day before summer vacation, I opened the textbook, carefully read about tenses, conjugations, and so on, and started to excel.
Now, on Instagram, I'm always amused by how some people sit there, reading words syllable by syllable and not being able to translate, despite their "school knowledge." But I find the material easy to grasp. Perhaps I could improve further now only if I went to an English-speaking country, as chatting and reading texts online don't offer much more at this point.