I was gonna tell the girl I like how I feel at a party and instead watched her flirt with another guy

I'm so used to girls liking other guys or not being interested that for ages I was kinda closed off and emotionally numb. I went in expecting the worst and wouldn't make a move of any kind on any girl I liked to save myself from failing. But I eventually met a girl who I connected with too much to just ignore it and put it aside. We got along far too well. I flirted with her at parties, I embarrassed myself, but I didn't care cause the crazy thing was she seemed to be reciprocating. She drew me in more and more and when school ended I decided I couldn't just let that be it between us. At this point I'm pretty sure she knew I like her. My friends have openly teased me about it in front of her, her friends know how I feel also and I'm sure they'd have told her. Not to mention all the flirting and whatever else that's went on. So, I started messaging her. At first we'd only talk like once a week (couldn't work up the courage to do it more often), but eventually that became every couple of days, and more recently it had become talking for hours everyday.

I knew I had a party coming up with her. She seemed excited at the idea of seeing me there, and I done the one thing I had tried so hard not to do for years; I got my damn hopes up. For anyone who's seen 500 Days of Summer, it was like the whole Expectations vs Reality scene. I stupidly thought this would be the night I finally win her over, where things would just click and magically fall into place. I was gonna tell her how I feel, and I was nervous but also excited.

Except, obviously, life doesn't work that way. Anyway, I found the girl alone at one point and I was gonna tell her how I feel. She looked down and I asked her what's up. She told me some guy she used to talk to had called her fat, and I just reassured her she definitely wasn't. She told me she knows, and that she doesn't even mind cause it all goes to her bottom half and makes her ass look good. I laughed and said no comment, but she looked at me kind of inquisitively and I ended up saying, okay, you have a good ass. She laughed, said thanks, and then spotted another guy who she told me she had to go talk to.

So why didn't I tell her at that point? Cause I realised I just got used for nothing more than a self esteem boost so that she could pick herself up and talk to another guy. The other guy in question is a guy who has her as is best friend on Snapchat, who's she's talked about to me before, and has always been good with girls. I thought there was something going on, and my suspicions were confirmed later on when I saw them flirting with each other like crazy. You know when you look at two people and you can just see the chemistry? It was like that. She was leaning in so far to talk to him she was basically resting her head on his chest, he had his arm around her whispering in her ear, etc.

I realised she used me to boost her ego so she could go flirt with a guy she really liked. And while I'm not gonna sit and act like one of those 'nice guys' who thinks just cause I'm nice to her I'm entitled to her liking me back, it still sucked. I felt like somebody sucker punched me in the gut over and over. It's been a couple of days since then and I've honestly just felt like complete shit. I've had no contact with her since the party, and honestly I don't even know if I want to. I mean I still really like her, but it's obvious to me now I'm wasting my time and that she's clearly not interested.

It just hurts though. I don't think I've ever liked a girl that much and I let myself believe she was into me too and things were finally gonna work out before it just blew up in my face out of nowhere. I just feel so stupid for thinking I had a chance, and like hollow. I've been in such a shitty mood, I can barely be bothered to speak to anyone and just want to sleep constantly. It's pathetic. I'm going out with my friends tomorrow and have no idea how I'm gonna be able to act normal and happy for a few hours. It sounds stupid, since we never even went out, but I was just so into her and I felt like we really had something and that she felt the same.

So, do I keep talking to her and trying, or would that be stupid? Will I just move on? And if I do, how do I get over her, cause right now I feel like complete and utter shit.