Why does my ex talk about his new girlfriend when I'm right there, it really hurts. Help?

so me & my ex started dating a couple days before christmas,& we broke up a few weeks ago. we had a great relationship at first,but the week before we broke up he started not texting me as much & not wanting to hangout as much.this made me have little trust issues,& i always asked him what was wrong & he would always say nothing.then one day i saw a pic of another girl as his phone screen,& we broke up that day.i tried so hard to work things out with him,& get him to understand how i felt.i even wrote him this really sweet note,but it didnt change anything because he went straight to the other girl.it turns out they didnt meet yet,but they were 'talking' through text,& i heard that he told her he was in love with her while i was with him.(she goes to another school & is older)so i havent texted him in like 2 weeks,& in school we just ignore eachother. hes in 2 of my cles,& today he talked about her so much. he talked about how they wanted to move in together this summer,& alot of other stuff.me & him always used to joke around & talk about how we would live together someday,so it really hurt me.he knows it hurts me & makes me mad,but he just kept talking about her with his friend. we have igned seats in that cl,& he sits right behind me so all i can hear is his voice,so theres no way for me to ignore it.i'm pretty much over him,or atleast i'm trying but everytime he talks about her inf ront of me it just upsets me & hurts me,because only a few weeks ago everything was different & he was mine. everyone says that i'm prettier than his new gf,& i'm a cheerleader & i always have to look cute(not trying to sound full of myself)so my friend thinks he talks about her to make me think that she is better then me,when shes really not.i think hes trying to upset me because i dont pay attention to him anymore.idk though.why does he talk about her in front of me, is there a reason? it just hurts. :'(