He had a baby with someone else

My boyfriend who I love so much that it hurts had a baby with someone else. Now this girl got pregnant 1 month before he met me. He told me this in the beginning but I didn’t take it seriously I mean his ex said she was pregnant but she lied so I honestly thought it was another girl playing a joke because he was a new relationship. Well 9 months later he tells me he’s at the hospital holding the hand of another woman pushing out a baby girl. Worst part is I’m jealous and it’s affecting our relationship. He says he been told me which is true. But now that the babies here it’s different when he goes over her house he’s there for like 4-5hours most newborns sleep all the time she isn’t even a month. They text and send cute little photoshopped pics of the baby. Now it’s getting to the point I want him to handle his responsibilities but at the same time I don’t even like when we talk about it cuz he doesn’t understand why I’m jealous. Maybe I don’t either. Am I just being selfish? I feel horrible