Don't know how to do it

So I've known that I'm transgender for at least 8 or 9 months now, and I've told most of my friends. I'm not out at school or anything though. But I have one friend who I'm very, very close to that I haven't told yet. I really really want to tell her that I don't identify as a girl, and that when I do, it makes me very uncomfortable and upset, but I feel like I'd be taking something away from her. We've been friends for about 4 years now, and we knew each other in elementary school too. She's always known me as a girl, and I just don't know how I can tell her that I'm not. She's one of those people who likes going dress shopping for no reason and loves to do nails. I let her do my nails and such when she wants to, and I come along when we go dress shopping, although I try to hang back a bit. My sister is also very close with her (we're twins) and is cis. So I know that she'd still have someone to do that stuff with, but I still feel like I'd be taking the person she knew away from her, taking the girl she's friends with away and replacing her with someone else. I'm still the same person. I haven't gone anywhere. The only thing that's changing is that I'm being myself. But I still can't do it. I feel awful for telling my other friends before her, even though she's my closest friend. I want to tell her. But I just don't know how I can.