Why am I jealous of my ex and his new confidante when I'm over him?

Long story short. I am in a very tangled situation where I live with my ex who I'm best friends with despite a very emotional breakup in March. I had a month away where I believe we both got over it...but, we are unfortunately stuck in the same house until June and after 20000 fights and failed reconciliations with one another, we have finally decided to try and be civil. Around two months ago we both met people who we became friends with. Ironically both of them are in the year below us. My person was a very very charming guy who needed my help with work and so became talking to me regularly online(this helped me even though I didn't plan on moving on) and at the same time, my ex became close with a girl who strangely had the same problems and became reliant on him to help. I suppose we both accepted we had new people in our life's that helped us think of even getting over one another. But here's where it gets ugly - the second my ex saw me with this guy(just casually talking in class) he became very bitter. Bitter to the point where I believe he scared off the guy. He gave him evil looks and took me aside infront of him one day and said "how dare you parade guys under my nose". He then, after a few hours admitted that he still loved me and so, stupidly I took my focus off being friends with this guy and began caring for my ex again. The guy still speaks to me but not nearly as much as he used to, this makes me angry and sad as we had ALOT in common. My ex's new " friend" though, has become codependent with him, and I see her falling for him and I'm pretty sure he likes her too. She texts him 24/7 and despite my jealousy I've tried to be nice to her as she is sweet, but I get a horrible feeling when they talk, a kind of apprehension where I'm scared that one day I'll see something I don't wanna see....he has a nickname for her on his phone and I'm angry because during our "trying again" phase, he wouldn't stop talking to her. I can't wait to move out and finally get out of this atmosphere but I don't know why I'm jealous when I know there is nothing left for us....