Masato, take a moment and think about how they feel. Put yourself in their shoes, and think about how silly your concerns are. They don't care, they've moved on (I assume), and so should you. That's easier said than done, but like Rex said, its a simple matter to distract yourself with something amusing. Just start doing something, keep yourself busy and don't dwell on the problem.

And Skippy, I feel your pain. Random depression sucks. If only internet hugs really worked... *goes to his room to sulk*