Why would a close friend get jealous when they already have a partner? Should I leave this situation?

I've been friends with this girl for a little under a year now. It started purely platonic but it got a little messy when she started initiating inappropriate conversations(her and her boyfriend had taken a break and that's when it started). We haven't done anything physical but we've sexted and done things on call, even while they've been together. I know it's immoral and I'm working on my self control.

My feelings for her have grown and it's made me realize I'm being stupid. I'm realistically never going to be with her, so I've started just ignoring or changing the subject whenever she does what she does.

Recently she jokingly got upset at me for not kissing her after a night out. I had the opportunity and I knew she wanted it because of the talks we've had, but I chose not to.

Anyways, I had another friend. One I'm not friends with anymore. My close friend would get jealous of her. So much that she would cry in fear that I'd leave her.

Thats not important anymore because it didnt workout with my former friend, but I do fear the future when I eventually meet someone new, either another friend or a love interest. While I'd love to be with my close friend, she's not available and I've accepted that. I admit I do get jealous of her partner but I don't act out or make her feel bad for it. I just want her to be happy.

Why do I get the feeling she wouldn't accept it if the roles were reversed? I really value her as a friend but it's a real concern of mine. Thanks.