Well, it's tough for sure. Reading your post made me think exactly of myself, no joke. I'm the exact same way and in the same situation right now with this girl who I got her number. The first time I texted her over break, I was sweating through my shirt just waiting for the response.
Anyway, while I do the same thing, I have improved a bit. I got really, really attached to this one girl last year. When I finally had to accept that it would never happen, I went into this deep depression. I literally altered my entire life for her. After that, I learned how to not get too attached.
My trick of getting over this whole mental attachment is to think about yourself and yourself only. It takes a lot of mental power to do this. But I am sure some of your thoughts are "what if she doesn't like me" and things like that. In this way, you are more concerned about HER. With girls, try to focus more on yourself. Make YOURSELF happy, ask "what if I don't like her as much?" That way, you relieve yourself of the pressure a bit.
I know it's so hard to do. And I feel like a hypocrite for typing this because even I do the same thing. But keep in mind, a girl will like you if you are confident in yourself. That means you need to focus on yourself. The reason girls don't like the stereotypical "nice guy" is not because they are "too nice" like they think, or that girls like bad guys. It's because the nice guys focus more on the girl than themselves. Try your best to focus on yourself, and YOU having a good time. Look what YOU have to offer her, not what she has to offer you.