F*cked up situation help?

I like this girl and ive asked her out and she said yes....thats all ok but the next thing you know after about of 3 hours of chatting and flirting i find out and i quote "She is in a relationship with so and so" *53 minutes ago* all of a sudden i cant brathe i start to sweat and i have the urge to KO someone or kill myself for ending up like an idiot