It's not that difficult to acquire fluent English speaking ability if you have learned English in schools intensely and you live in some English speaking country for a month or two. But, the fluency does not always mean correctness and naturalness. Those who speak English fluently and have no difficulty in everyday communication in English do not always speak natural English like true native English speakers.
I hear that a coach of a team of Premier League in England who is actually a spanish speaks English really fluently with almost perfect pronunciation, while a scrutiny on his English reveals that he never use past tense.
Think how long you have been speaking, listening, reading, writing Japanese, how many books you have read. To gain fluency in English, one of the best way is the exposure to English. Put yourself in such an environement in which English is exclusively spoken and written. through this kind of experience, you would develop a kind of your internal nerve system adjusting to English language.