I have no idea where and why the jealousy comes from. help?

I used to date my best friend and we became roommates. Outside a lot of miscommunication and I dealt with trauma, we separated in a toxic way. We still live together but I move out next month. Anyways, he had this female friend. Day one there was something about her I didn’t like. She disrespected me multiple time in my own house and admittedly I let it happen because of my own lack of self esteem and not being confrontational. He ended up hanging out with her more and more and brought her around even though I asked to limit our contact. He put more effort into her even when we were still romantic. According to him and our friends, they never got to have sex cause she used him for oral purposes and never returned the favor. Hilarious tidbit of info cause her whole motto is “I don’t fuck my friends.” He was cool with eating her out until he messed that up. They’re still friends to this day. I’ve been chilling at our apartment for four months alone. Yeah, it hurt getting replaced but I can find better. I am still kinda jealous that they can just hit it off with convos while we struggled for the longest. Even now with the strained distance between us and him not being home, it sucks talking. She’s changed him. He has leaned into a big idgaf attitude but I can’t help but notice how he says he doesn’t care about something when it’s clearly bothering him. I tell myself that it’s not my job to save another person from their demons. I don't know maybe my feelings for him haven’t fully left or maybe I see her as someone I just don’t like. Outside of playing men like some men play women, that’s the main thing I don’t like about her. Why use someone else’s feelings for you to your personal gain?
I have no idea where and why the jealousy comes from. help?
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