Why are some dudes just trifling and fucked up

I am 7 months pregnant and I find out that the father of my child has been creeping with his other baby’s mother! I’m literally sick to my stomach. I just wanted an explanation and he tells me “it’s complicated and I won’t understand”. My thing is.. why bring a new baby into this?! We already have another child at that. This is all my fault though, because I shouldn’t have taken him back when he cheated on me and had a baby with her in the first place. I’m literally broken