Am I being Irrational? Is my anger justified?

So my ex and I were dating for a year and a half. He was the first to tell me he loved me and even then I didn't say it back for about a week or so. Throughout the relationship we would have conversations about things that bother us, but eventually it was the same conversation every time. I would always take the conversations serious and try to work through it. But he never seemed to try at all. There were times he almost left, and I did NOT ask him to stay, but he did and wanted to work it out. He even begged me not to screw him over and break his heart. Well at a year into the relationship I got pregnant. He was not happy at all, but again stayed. He waited until I was six months pregnant and left, but cried like a bitch as he did so. When our son was born he was there and has honestly done his best to be there for him. But I recently found out that 2 ish months after he left he started sleeping with this girl, a month before his son was to be born. After month after our son was born they started dating (October is). This man comes over to my home almost every other day to spend time with his son and treats me like he is still "Trying to find himself" or "Figure out why he feels he doesn't deserve happiness" giving me hugs before he leaves and doesn't tell me until Christmas that he is saying someone... And had been since October. The girl he is dating is someone who early in our relationship he used to talk to for advice about him and I. I told him then I didn't like her and wasnt comfortable with that because I just had a feeling about her. He told me I didn't need to worry about it, she was married. Well, come to find out about the time he started acting like an ass in our relationship, she seperated from her husband. He claims he wasn't looking for anything and "It just happened". I call bull shit. The more I think this the more Angy I get. One, I feel like this hoe has been after him all along. Two, it almost feels like maybe he did cheat on me. And Three I find it incredibly disrespectful that all this went down, while I was pregnant and sick and trying to then recover from a very rough pregnancy and he is acting like nothing is going on claiming he'd do anything if I needed it and that he still cared about me as the mother of his son and what not. But also states he has had doubts about our relationship from the beginning. I call bull shit again, he is a stickler with his credit and his money. He wouldn't of co-signed on a car with me if he had had doubts. I just have all this anger and rage.