There are a few things I can say here. First, concerning the confidence. To build up confidence(I mean permanent confidence, not temporary) do something to improve yourself(working out is the only one I can think of. Of course, there are other things out there I bet). After you have improved, you will begin to get more respect for yourself, increasing your confidence. For me, I work out. Not for muscle, or to impress the girls. I do it to be strong enough to hold my own in a fight but that's a different story. Anyway, I feel better about myself.
After this, meet people. Just meet more people, make more friends, and your confidence will improve. Approach more girls to get numbers or e-mails. You will be rejected some times. It's inevitable, so don't freak out about it. I'd give you some more advice concerning long-term confidence, but I would need to get a look at you.
Now, concerning this girl. I can already tell what your biggest problem is. When you, like many men(nerdy or otherwise), speak to a woman, you have this thing of wanting to get approval from her. That makes you look pathetic. When guys who "get it" approach a girl, they don't look for approval. They approve of , and they have friends who like them, and that's enough. When they approach a girl, they want to find out if they are worth THEIR approval.
You don't have to act like you need her approval. My theory: In fact, it is the males natural position to do the approving(though the girl can say yay or nay to guy). If it was the other way around, it would be the guys getting all prettied up and spending ungodly amounts of time on their outside and girls would be the ones just throwing on a shirt and a pair of pants and be satisfied with that.
If you get a conversation going with her, do not talk to her more than 20 minutes, if even that long. Talk to her a little(in which you do not demean yourself in any way, shape, or form. Even if you're just joking) and then grab her phone number of e-mail. If I were you, I'd get the e-mail. It allows you more time to make some kind of message and/or date without coming across as a wuss. It's harder to do on the phone. Also, when you are talking to her, tease her about her insecurities and mistakes. Not maliciously, but in a playful manner. And the most important thing to do is not to put her on a pedestal. She is not superior to you in any way. She may even be inferior.
I have more I could type, but I'm not going to waste my time because some of it is case-specific and I don't know the particulars about your situation. Feel free to give as much info as possible.
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I am the god of dating! You will respect my authoritah!
Democracy: rule by the stupid