Hate that I'm feeling this way

So me and my ex split up about 8 months ago. Since we broke up and probably even before we broke up he has had a relationship with this girl. He kept saying they were just friends but I knew he was lying to me. Tonight he finally acknowledged that they were more than friends. We have a daughter together so it really bothered me that she was spending so much time with her considering he couldn't even be honest about who she was to him. I dont want my daughter around guys or women we aren't serious about. Anyways I can't stop feeling sad that he couldn't try more with me for the sake of our daughter. I know I would never want him back but it still hurts a little. He always had a cheating issue and who knows maybe he will or has cheated on her but what if he doesn't with her? It would make me feel like I was nothing to him. Again I'm not looking to get back together with him. I know my worth and he never treated me with an ounce of that. Maybe I'm being petty but it would have been nice if I was the one that found someone first. I mean after all he was the one that said he can't do relationships and that dating wasn't a priority because he wanted to focus on our daughter. I just don't get why it was so easy for him to walk away from the family we built so he could go off and be with someone else and only want to be a part time parent. He doesn't even check in to see how she is or to talk to her when I have her. It's like she doesn't exist to him when he doesn't have her. Don't know what I'm looking for in this. Guess I just needed to get my feelings out