I used to feel like this once in middle school, about
this Asian girl from my journal... I had never kissed a girl before, or even tried doing any moves towards girls, other than being the usual neutral gender-ish nice guy. Then I confessed my "love" (honestly I just wanted to fuck her, she had a great ass and I was into Asians) for her in front of the whole class on a little birthday party we had for her on recess. It was awkward as fuck, and I would always get jittery around her and barely able to speak, but, with time, she kinda rejected me, on a nice girl type of way, like "ooh let's be friends", then we have nothing in common, so of course we can't even be friends lmao
After a while, I kinda ignored her a bit and it went away, slowly.
I don't know how old are you, but you gotta see women as people just like you and me.
I think this type of nervousness around that one girl comes from masturbating about her in your mind (and many times, idealizing her), like thinking too much about that one girl, and when you see her, it's this big deal, like your biggest objective in life, right in front of you, so of course you get nervous.
I could also be that this girl really is special and messes with you in some cool way (though the masturbation thing is kinda real, it probably still happening, right, because if you like her that much, you will be thinking about her). Then, you should do your best to calm yourself down and get her, with your best game. It's gonna be an uphill battle, because your emotions are gonna make you needy and over the top, but it's possible to get her still, if you're used to dealing with your emotion (I managed to fuck
this old crush of mine from uni, but afterwards I got a bit needy, fucked up on the next date, and never saw her again... still it was fun getting her, after being so into her years ago)
I also felt a bit about this young girl I made out like two years ago on a uni party,
when I saw her at the club, a few weeks ago, though still I didn't bed her (I think it's possible, but again, it's gonna be hard, because my emotions make me act kinda silly around her, and I gotta override that somehow).
I feel now that I'm on a relationship, women are scarcer to me somehow, because even if girls show they are very attracted to me, even more than before (mostly due to me improving my fundies and game, but also giving less of a fuck about other girls), I don't wanna hurt my girl, so I'm only seducing girls on the side, when I can, so the more women you have at your disposal (for sex, of the quality you want/think you deserve), the less you feel like that.
If you don't want your emotions to be messing with you, aka you wanna stop feeling like that around her, the best thing to do is forget about her completely for quite some time, and go sleep with other girls. Eventually, she'll be just another girl to you.
And deff read
that article @Kaiderman put there, it's a very good read, and explains and sums all of that up.