It's "coming out of the closet". It means to finally admit publicly to a secret you've had for a long time. In most cases this is used when someone declares that they are homosexual. It comes from the earlier expression, "closet ____". So in your example, you mean you were a "closet Harry Potter fan".

Are you really sure that is the way you want to describe yourself? As Peachey says, it has other meanings.

No, what she says fits. Coming "out of the closet" is related to homosexuality, etc, but coming out the cupboard is better fitting to a Harry Potter fan because Harry used to live in the cupboard under the stairs. Apparently yall didn't read the books. You can say you're coming out of anything and it'll still mean you're revealing a secret. There's nothing wrong with that phrase.

harry lived in a cupboard? lol No wonder he is the way he is. :P