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-we end up fighting over all that, and i eventually find out shes seeing this guy i really dont like, so i tell her off once and for all and 2 days later they officially start dating.
I completely know the feeling here. Almost the same thing happened to me two years ago and I can honestly say I wish I handled everything differently looking back on it all. I've most definitely done some growing up since then.
Back then, I would have said to go to the party and
make her jealous. Right now, I wouldn't do that at all. If you have bitter feelings about it (who wouldn't feel a bit hurt), you associate her with a negative current situation. If you make her the focus of your night, it will also have a negative outcome. Negative intentions draw negative vibes, and there is no need for that. Instead, the focus of your night should be enjoying yourself and welcoming others into your fun.
If I were in your position, I would go to the party with the main intention being to have a great time, meet new people, and socialize. You may end up meeting a cool girl there that you really hit it off with. I don't know how your feelings are towards her right now (as in bitter or just don't care anymore), but don't go for the purpose of getting a reaction out of her. Go to the party for you and just vibe.