When I read your first post, and the word Sertraline, my first reaction was, he'll find another woman, and sadly reading your last, this has happened. I was married 28 years, divorced just before our 30th - I had to divorce him for my financial safety, and you will have to look to your finances smartish. And still my husband pretends she's not in his life, even though they've been together 18 months. I am not a health professional but from what I saw with my husband Sertraline sent him into a constant hypomanic state, he matches every symptom. Mine lives in a fantasy world, not far enough removed to be unbelievable, and even I would believe some of the things he says if I didn't have the evidence otherwise.
You have 2 things in your favour, firstly he's moved in with this woman, that means everyday reality will invade, whereas mine in his words sees his girlfriend for sex and convenience - and he spends shedloads of now borrowed money on her as part of the mad party. More importantly, yours has agreed for you to come to support sessions with him. I could have, and should have insisted on being a part when I had a chance. A friend who did, and fought tooth and nail to continue to be a part of the counselling, went through hell, but is now very happy with her husband, who is now on effective medication and although much is a hazy memory has been helpful to me in giving insights. Be braver than me. Right now he isn't your husband, something is sparking out in his head, but your husband is in there.