I think you shouldn't bottle it up. I'm a girl also, (although it may not seem like it:3) and I asked my crush out, and now she's my girlfriend. It doesn't matter if she doesn't like you or not, and if things don't work out then I guarantee that you'll find someone just as special that loves you back. If you bottle it up, then it will just make you more nervous overtime. But I know you can do it, I believe in you, and so do all of us.
Just take a deep breath, relax, and tell her how you feel. I know it might seem hard, but you can do it 😊