How do you move on?

My husband and I have been together 13 years; June would have made 11 years married. Recently he told me that he’s not happy and he doesn’t want to be together anymore. He’s even sleeping on the couch. I’m completely heartbroken! I totally didn’t see this coming. I’m 90% sure he’s seeing someone else but I’m not one to go through his phone. But he’ll go away for weekends without me or the kids, saying he’s visiting family or friends. If I ask to join he’ll say no other wives or girlfriends are going. To make matters worse I’m currently 12 weeks pregnant with our fourth child (that was planned btw). I honestly dk what to do. Part of me wants to fight for my family, part of me wants to just let him go. I never imagined raising my kids alone or being with anyone else. He was my whole heart. I want nothing more than to make things work but I know I can’t make him want the same. 😢 How do I move on??