That's the whole issue in a nutcase, really. I'm perfectly fine talking with girls that I don't like, girls I like, I have problem talking to.
Sort of the same problem here. I can sing decently. But when I'm around my girlfriend, my singing sucks. Hehe
In any case, I can't remember how it was waay back when in 6th grade. But I think I was in the same situation as you when I met her. Maybe. What I did in that situation was to try to get to know her a bit better, know what she likes, dislikes, and what makes her laugh, then work with that. And that's where I am now with her.
In any case, follow what others said. Try to go with the flow.