Why is this happening to me??

I just had my 1st baby Jan 15, she's beautiful and perfect and I've never loved this hard before! But I'm so incredibly hurt because I just found out my SO has been cheating on me with his ex, the mother of his other 2 children. They've been sending each other nudes and stuff and the kicker is..... my baby was only 3 days old and we were still in the hospital when he was doing this!! And again 3 days ago!! I wish I never looked into his phone but I'm even more disappointed, disgusted and depressed that he would do this to me and to our little girl. I don't know what to do now because I thought he was the one.... 😢😭😭 If I stay I know he will never stop because they've clearly been doing this throughout my entire relationship with him and it's not right to be teaching my daughter that it's ok to be treated like dirt, but damn it I'm scared! My whole world literally just flipped on me in a matter of seconds. I am hurting so so much 💔💔💔