Falling apart

My relationship seems to be falling apart. I've been married for 2 years. Been with him for 8 years. Everything seemed great. We went thru infertility and when things seemed down we found out we were expecting. I am now 18 weeks expecting an amazing little boy. But instead of being super happy I'm so depressed. It feels like I'm going thru hell. I'm loosing my mind. I'm so sad all the time. He says he doesn't know what he wants. Doesn't know if this is all worth it. And he told me he was talking to someone else. I don't know what to do. I feel like I'm dying inside. And I have no one to talk to. My family is breaking and I don't know how to fix any of this.