After some time in Japan I had similar feelings. When I first got there everything was so exciting, but in time, nothing seemed to interest me and I wasn't motivated to do much.
This may not apply to you at all, because your reasons for being in Japan may differ from what mine were, but after this feeling persisted for some time, it was a sign to me that it was time to go home. BUT, that's because my sole reason for being in Japan was to have a laugh. When the feeling died, there really wasn't much reason for me to stay there - although I still love Japan. Don't take this as meaning that you should go home. There are some options....
There are some things you can do to combat this feeling. GET OUT OF THE CITY!! I lived in Tokyo for a while and took a rafting trip in the country with some friends, and as we were taking the train out of town, the further out we got, the more my stress level decreased. You don't realize how uptight the city is making you feel until you take a break from it and see how much better you feel. But, that might just be me.
Also, get involved in some activity. Maybe something not all that Japan-related that you'd still be interested in doing if you were at home. Like, play soccer, learn to play an instrument, take Japanese classes (J-related). I always get depressed if I have too much free time on my hands.
Japan is a place just like everywhere else is a place, and after the initial thrill wears off you're going to have to take some steps to keep your sanity, just like you'd have to do anywhere.