Some good points made here. I think a lot of guys, and for that fact, girls too can take a lot of this advise into their own lives.
If it was me, I would just let it go and move on. I know it kills to let go, I've been there and done that like many others. But we have to be realistic here. What are the chances of you adding the girl, then her being nice to you, then the whole thing getting to a stage where you're both more than friends? Wouldn't you think it's creepy if a guy added you like 2 months after not actually meeting her and then wanting to be more than friends?
Do you think she would really want to meet up if you ask? Chances are she won't, but there's still just as good a chance she will.
I think this all comes down to each others personalities and such. Seems like OP doesn't have the confidence to do that.
Let me tell you all a little story about myself... I started my first day of year 11 and it was also the day I met 3 new girls (they were basically their own group of 3). There was one who was in one of my classes and the other 2 weren't. One of the ones that wasn't in my class was my biggest crush of my life. I did have a crush on the one in my class but as I found out more about her, the more I got turned away even though she was such a nice person, the things she did and still did were something I couldn't live around. Now, the one that I had the big crush on was someone who I didn't really think much of when I first met her.
When I started year 11, I was actually on a "break" with a "girlfriend" (it was complicated as hell). When we were "together" I felt as if my life was limited to just her. I stopped everything to talk to her and we only saw each other about 12 times in the year (lived far away). I felt so locked away from my normal life... So when her parents had a talk with her (after MANY talks) I couldn't take it. They didn't like her dating so I told her I couldn't do it after about a year. So when we were on this "break" I felt so FREE. My normal life was back. I could talk to other girls without feeling like a jerk. So after about 2 weeks of year 11 I told the "girlfriend" I wanted to end this. So I did in a very kind way. I explained to her WHY I couldn't do it anymore and blah blah.
So after that, I started talking to the crush at school a lot. We'd sit together during breaks and all that cute stuff. Sometimes she would even sit next to me when she saw me by myself. Now I think back to those times, I really regret not asking her out even though we were such good friends in such a short time. I don't know why I didn't think of asking her out before, but when I did, the SAME week I thought about it, I found out she had just gotten a boyfriend. From a different school. Lucky for him cos I would have punched the donkey balls out of him. Just kidding...
So I thought I'd lost her for good, so I just naturally cut down talking to her cos I thought I was going to steal her from her guy. Then when she broke up with her I had pretty much stopped talking to her already. Anyway, I still think about her till this day, but It's not as bad as OP makes himself seem about his crush.
Moral of story... Guys, Girls, If you see someone you like, just go up to them as soon as possible and introduce yourself. If it's school then you can put it off for a few days and just flirt via eye contact then introduce yourself. If it's something like OP's party and you know you won't see her again, then you have to man up and do it then and there.
Sorry OP.. I know how you feel.