What is the most difficult part of learning English?

I would say doing it every day. It seems easy to learn English when I see that kind of stuff with script or guide. But it's like solving math problems. I have no idea when I first see a difficult problem, but after checking the answer sheet, I understand. The same goes for English. Just knowing a word, sentence, and grammar theoretically and speaking or writing English on my own is totally different. To overcome it, I think that continual practice is the only way. Even though I'm not sure that my English is going to be better, the most important thing is that I believe it can be better, and practice it every day.