Well, learning the basics in school wasn't much of a nightmare: English is a compulsory subject here, so I've been studying it since I was, like... 9?
The grammar was never -that- difficult to me... but then again, my native language is Italian, we have verbs with suffixes that change with each pronoun, we have a ridiculous amount of articles and we randomly assign genders to inanimate objects, so I guess my perspective on what's meant to be difficult is kinda skewed 


What WAS difficult, though, was learning stuff like slang and chatspeak. I found that the best way to learn quickly was to spend time on forums and chatting with other native speakers... however, not everyone I talked to seemed to be aware that slang terms, abbreviations, acronyms and chatspeak can be VERY difficult to understand for a non-native speaker. I wasn't aware of the existence of Urban Dictionary back then, so when I saw people saying stuff like "LMAO", "ROTFL", "BRB", "wut" and so on and so forth I used to get extremely confused XD false friends were also another can of worms of their own. Even to this day, there are sentences I know how to use in an English-speaking context... but have absolutely NO IDEA how to translate in Italian :'D
Pronunciation was/is also incredibly hard and one of the things I'm still very self-conscious about. The fun thing is that, despite the fact that my entire school career was focused on learning British English, I have a much easier time understanding American English. Though I guess Youtube, videogames and in part movies are to blame for that :'D