I am SO freaking glad I don't normally get jealous, and when I do it's a small and momentary spark and nothing more. It sounds miserable.
I suspect the jealousy is based on a feeling of a lack of control, and issues you have with yourself moreso than the girl or anyone else there. You're getting a lot of advice about what to do in relation to HER, but I don't think that's the real problem here and I think you know it, too, judging by your post. I'm not sure why everyone else ignoring that, but...
You need to try to figure out WHY you feel jealous, I think. You say it feels like everyone is against you, even though logically they aren't. Is it just a feeling of competitiveness, of them being obstacles that are getting in your way, or do you suspect them of intentionally sabotaging or attacking you?
Maybe the jealousy is an excuse your brain is bringing forward to excuse your own lack of action in this situation. But then, plenty of people feel jealous despite acting. It seems to be related to issues of insecurity and fear. So are you afraid of something? What, exactly, are you actually afraid of, that is triggering the jealousy issues? That one of them will swoop in and take her before you have the chance?
I'm not going to pretend to have any real answers, of course. But hopefully asking these questions can help you get a bit closer to figuring it out.