I don't know how old you are, but I am sure that your English is at certain level (because of education in Korea :P) and how each word sounds like.
In my opinion, the reason why you cannot understand spoken English is because each word is not separately pronounced as if they are written, but the features of connected speech occur and sounds change a little bit, which is quite normal for many other languages I would say, because even though we do not notice, in Korean 같이 있다 -> [가치이따] (something like that :P).
So that was my long reason why it is difficult to hear natural speech of English, and to tell the solution, well, I think if you watch some English dramas (American, British etc.) and try to do some dictation tests without looking at the subtitles, and then compare what you wrote with the subtitles, I think you would be able to slowly catch the features of connected speech and improve on your listening. Oh, and I don't recommend to do dictation with CNN or BBC news because first, there speech is normally slower than daily life speech, and second, they are not very insteresting and difficult.
Hope that helped!