How to stop liking this girl?

I'm in high school and there's this girl I've had strong feelings for for the majority of it. She'd been dating the same guy since middle school and although they've been on and off many times I didn't pursue her because I didn't think it was the right thing to do. It's pretty common knowledge among even my not-so-close friends that I have feelings for this girl, but I'm sort of the jokester of the group so people don't often take me and I guess my feelings for this girl that seriously. A few months ago she broke up with her boyfriend for good and one of my closest friends (who i guess didn't know about my feelings for this girl) pursued her immediately. I still didn't go for it because I didn't want to take advantage of her, even if it was inadvertent. I thought their fling would pass but it's been more than six months now and they're essentially in a relationship, although they won't call it that. I used to dote over this girl all the time and now she expects me to act the same way, but I only feel sad or resentful when I'm around her and her (basically) boyfriend so I've been trying to avoid them in the most gentle way possible. Over more complications they both found out I have feelings for this girl so I'm constantly feeling like they're censoring their relationship for my sake and just patronizing me in general, which just hurts even more. I just want to get out of this weird love triangle I'm in because it's gotten to the point where I can't help but be a jerk to them because I'm so resentful that they care so little about my feelings. Please if anybody can just explain how I can get over her or maybe point out flaws in my actions and logic that I can change in the future. I really don't want to ever be in this situation again.