I’m looking for advice on how best to help my husband. 
My husband (33) and I (34) have been married for close to 7 years, together for 10. We are best friends and very much attracted to one another, but four months ago my husband told me he wants to separate. I was shocked and devastated. 
The more we talked the more I realized he is depressed. I remember as teens he was depressed, but no steps were taken to get help back then, and not long before we began to date he seemed “over it”. But it’s returned with a vengeance. 
It’s very confusing because he says he loves and cares for me, but doesn’t feel “in love.” There has been a lot of intimacy still, though, and we get along fine for the most part. He will only say that he wants to be alone (move out) and that he senses a darkness in him that he doesn’t want me to be exposed to. 
I’m trying to respect his wishes and give him what he wants, but I’m scared and worried. It seems like the time he spends alone is the time things get the worst for him. When he has his own place I won’t know if he’s okay and it may be more likely that he isn’t okay. 
I should add that he did start antidepressants a little under a month ago, but only negative side effects so far. 

Can anyone relate to either side of this?
What can I do?