How do I tell my girlfriend I have vitiligo (skin condition)?

Anonymous
We have only been dating for about a month and a half but I really like her. We are both really nerdy and have a lot of the same interests. Anyway, I have vitiligo and it's really only on my groin so she doesn't know I have it but I know I'm gonna have to tell her before we do anything sexual. We have been making out a lot and she tries to go further but I have stopped her a few times now. She's been with two other guys but I've never done anything sexual with a girl because I'm scared of how they will react.
No one outside my family knows and I've never told anyone before, I really have no idea how. I am scared that she will completely reject me.
How do I tell my girlfriend I have vitiligo (skin condition)?
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