Dear Mayumi.
I've read your thread repeatedly and I confess I have the same problem as yours.....the more I study and take note of unknown words the more insecure I feel about my English level.....but you have a huge advantage over me because you must live in Tokio where there are certainly a huge presence of westerns so it's easier for you to have conversation with anyone.......as for me, I haven't had opportunities to do the same because I live in a non-touristic and boring town where it's quite difficult to come across foreigners even though there are important factories and other businesses.
There's no need to be bored and hopeless....don't push yourself.....studying any language is juggling.....studying two languages and being such a disorganized guy at the same time...yeah, this is me!!!
Finally, dictionary has been my main tool, my "oracle"......every time I type any article or reply to these threads, my fave dictionary is:
Cambridge Dictionaries Online - Cambridge University Press