Okay, so I was never actually planning on doing this, but I really trust this person and suddenly it felt right to actually tell her. I hope nobody rushes into telling anybody for it to all go drastically wrong...
Anyway, it went GREAT . I didn't actually tell her, I just linked her to understanding.infantilism.org, and let her read it, and she was so great about it, and she hasn't just "accepted and never speak of it again" she is asking questions, and she says she isn't freaked out at all.
I can't believe how great it feels to be able to tell somebody and have them accept it: accept me for who I am. I can't wipe the grin of my face XD.
Just thought I'd share