How could he...?

I know this isn't a place to rant but I feel like I need someone to talk to. My partner and I aren't formally together but we have intentions to be a family, us and our child on the way. After our relationship seemed to be going well, he decided to get a girlfriend. Now I'm hurt because I really wanted us to be something but I'm more upset at the fact that he couldn't wait until the baby is here to seek a relationship. I know I can't tell him what to do but I decided to distance myself away from him because I feel overwhelmed when he's around. He's priority is now on his new girlfriend. Am I overreacting? Should I not be upset? 10 weeks pregnant and alone in this.