The people telling you this are full of %$#@. Just because you aren't seeing flying manatees and want to kill everyone who ever insulted John
Travolta's dog doesn't mean you don't have a mental illness.

Depression is subtle and vicious. It often works without outward signs, devouring its victim from within.

As others have said, you do fit in. There are plenty of people just like you and plenty of people that are willing to understand. Maybe you haven't found them yet, but they're there.