My ex cheated on me and they’ve been together for a year now

This is such an odd thing to admit online. I’d like to start by saying I’ve been trying my absolute best to focus on myself and really let go. I just can’t wrap my head around the fact me and my ex have been on and off since middle school. We’ve been best friends and romantic partners for 7 years. And three days after he boasted about how much he missed me and loved me blah blah he wanted to marry me, he broke up with me thru chat gpt. I later found out he cheated on me and was still with the girl(a FRIEND I had). I don’t know why he doesn’t care or feel bad or upset or anything. He’s in a happy relationship currently and I’m still here single and sad over him. I’m prettier than the girl btw so I’m just so baffled that he’d do such a thing and never tell me even when he was trying to break up with me. Was he trying to leave the door open? He broke up with me because we honestly did fight a lot because he was too ‘busy’ to the point he couldn’t text me Goodmorning and goodnight. That’s it. Not even a conversation or anything all I wanted was a greeting from him. But he had enough time to acquire a girlfriend. I’m starting to wonder if that’s the reason he was ‘so busy’ if anyone has theories on why he did this I’d love to hear it. Especially from another man because I truly amm stumped