How can I get rid of this jealous feeling, and stop feeling like a bad friend?

I'm 16, 11th grade. OK, there's a girl I've liked since like 9th grade. I asked her out and she said no, so I was like "alright fine whatever, your loss". Then I see her with one of my good friends I've known long time (since like 1st grade), and got super jealous. For some reason I kept picturing him banging her after prom (and her moaning at him) whenever I'd see them, because when I was 9th grade some seniors from my baseball team told me prom was just a misspelling of porn, and everyone bangs afterwards, and it gave me a headache whenever I'd see them, so I basically just avoided them so I wouldn't end up doing something dumb. I wanted to swoon her over so bad, but I can't do that to my friend, and I feel like such a bad friend because of it. Not to mention she already said no anyway, so why am I getting so damn jealous for?