AITA for telling my boyfriend to get over it and that his ex never loved him?

I (18F) have been with my boyfriend (18M) since we were both 16. He had just pretty freshly left his ex at the time of us getting together, something I didn't know until about a month in. During the beginning, it definitely caused some conflict because I was extremely insecure and he wouldn't stop talking about her. We were 16, so of course I let it slide and of course he never shut up.

About 3 months in, I got pretty upset and I finally opened up about it. I told him that it made me really insecure and that I felt very disrespected. After that, he was much better about it but I couldn't stop thinking about how he probably still thought those things. Here's the thing though--I don't think he still loved her. I think it hurt his ego. My reasoning: He would say things like "I can't believe she's with him." and "Why does she think I'm the bad guy?" Things that scream to me, "Ouch, that hurt my ego." I guess it was also just the way he said it and the face that he'd make that makes me think this way.

Some context: I don't want to be the jealous new girlfriend, but I genuinely don't think she had feelings for him. I used to be good friends with her now best friend (before I even knew who my bf was), and apparently she would constantly make fun of him and say things like, "I just like having a boyfriend, idk." This isn't confirmed, though. Also, just based off of what my bf has said, she used him for his body. I also can't confirm this, It's just what I've heard.

Anyway, here's the main event! We had prom recently and by then we had been together for just under 2 years. Me and him were having an amazing time together until toward the end of the event. He noticed her dancing with her new boyfriend and he would not stop staring at them. At this point, I was certain he was over her. Obviously, this upset me a lot. We've talked about this before and I was finally feeling secure with him. I confronted him about it and he said, "Sorry, she's with the guy she told me not to worry about." (Another thing that makes me think its hurting his ego?)

Even after I confronted him, he wouldn't stop. Also during this time, he really wanted to have a romantic slow dance but obviously I was a little angry and kept saying no. He got mad at me and asked why I wouldn't dance with him and that's when I blew up. I told him that he'd been staring at his ex for the last half hour and that she never cared about him so why does he still care so much? He was silent after that for a minute before he sat down and apologized, though it wasn't genuine. It was snarky.

After prom, he dropped me off at home because I wasn't in the mood to hang out any longer. We've hardly spoken since then except for a good morning from each of us. AITA?