Can't stop having intrusive thoughts about ex-bf

bf and i broke up a month and a half ago and i found out that he started talking to a new girl three weeks into the breakup, talked for two weeks and then began dating. we've talked since and it's become very clear to me that this is most defiitely a rebound relationship but i can't stop thinking of him with her. i was his first girlfriend, and we were each other's first everything basically. the thought of them sitting in his room laughing or kissing or being intimate makes me physically ill but i can't stop thinking about it. i know im just ruminating and in time i'll stop but it feels awful. we were together for 4 years and so i know this man inside and out, i know what his eyes look like when hes in love or happy, i loved his laugh and the way he would hold me. but now, i just get upset even trying to hold on to the good times because all i can think of is him doing the exact same stuff he did with me and its making me feel like i didn't mean much to him. my friends are saying that its a rebound because of the timing of it all and the fact that he still reaches out to me here and there. not to talk or have in depth conversations but he's texted me a few times about things that didn't matter (and this was after i found out he had a gf- he told me- and after he blocked me on everything but messages). im just confused because it feels like he still must have feelings for me, yet hes offically dating another girl. am i crazy? am i being delusional for feeling that he might still love/like me? and how do i stop thinking about them together?