Hi Genevieve, welcome to the forum and sorry you had to find it. Neither us or you can diagnose your husband, but I will say that from everything I have learned and experienced myself the sudden shut down of feelings is a hallmark of PTSD. My ex-girlfriend was madly in love with me one day and the next time I saw her she talked to me like I was a stranger. It was like talking to a robot. If you haven't already, read about the Stress Cup. Intense emotions unrelated to his love for you or other life stresses could cause his brain to shut down all emotions, even the good ones. His brain is doing this to protect him. Think about someone so overcome with anxiety and despair that they are on the brink of suicide. Their brain, always concerned with survival, shuts down these overwhelming emotions -- which is great, because it may have just saved the person's life. The problem is it's not just select negative emotions that are turned off, but all of them. This can make the sufferer feel like they are no longer in love.
Try to put yourself in that situation yourself. Imagine you all of a sudden no longer feel love for your husband. When you imagine this, you probably imagine you'd have the self-awareness to say to yourself, "Hmm, that's weird. I SHOULD still love him. Maybe there's something wrong." But that's expecting a lot of anyone in the throws of mental illness. The truth is if you felt something or didn't feel something, it would be absolutely true for you. Your husband can't feel love right now, so to him it's logical that he's not in love with you. No love feeling = must not be in love anymore = must end relationship. Makes sense, right?
I can't say that this is true of your guy, only that it is a possible explanation. What happens next in your case will be different from what happens in others. As much as we want to come to this board and get hope, no amount of explanations or stories (encouraging or discouraging) on here will shape what is to come for you and your marriage. For you it's going to be all about riding the waves of this storm, surviving long enough to see where it's going to put you down. And that means taking care of yourself. Sadly, your husband can't give you the love and reassurance you need. And you can't help him. He has to want to help himself. It's good that he is still communicating with you -- you'll see many stories on here of partners who suddenly fall out of love and bolt.
Keep us updated and, again, I'm so sorry you're going through this. Mental illness -- be it PTSD, depression, or anything else -- is awful to experience for both sufferer and supporter.