Japanese are good at English (I think) maybe it's hard because of cultural differences - Japanese are not used to saying "you" for example. But it is easier if you can live in America for some time and then your English will become very good. Good luck!

You should hear some Chinese speak English even though they may be fluent in the language.

I think Nico is right. The structure of the two languages are so different that some things are difficult to internalize. どうして日本人じゃない人は日本語を話すのが下手ですか？

I don't think Japanese are poor at English. You should hear some Europeans . They speak awfull. The thing is that English and Japanese are very different in grammar ; and in the way of speaking. But I think you can learn English better than you think. :)