Is my jealousy rational?

Hi Everyone.

I started dating this guy about 3 months ago and everything was going great.. The only problem is, I have become too jealous and it's eating me alive. It's literally making me depressed and I can't think straight. He has an ex girlfriend that he broke up with about 6 months ago (they lived together) and he still talks to her and even hangs out with her sometimes. When we were just friends he told me one night that he was thinking about banging her just to get back at her current bf, whom she cheated on him with.The issue I have with this is that she is very flirtatious…with every guy. She's the kind of girl that grabs your tie and pulls you close to her in a casual conversation. It's really weird. And it makes me uncomfortable. Another huge issue I have is that we go to this bar that we kind of know everyone at. He has another girl that he used to mess with that is always there and she's always throwing herself at him. Sometimes I catch him looking at her and then he always quickly looks away. At times he will even throw a comment at her. I have a problem that she has never been introduced to me and she always gives me bad stares. He has so many exes in his life and it confuses me. How do I stop being so jealous? And do you think he still has feelings for either girl?