My ex was off secretly seeing other women, but at the same time he didn't want to break up our relationship. The idea of such a big change after 20+ years, and a change that would be his "fault", was very stressful for him. He even told the last OW that maybe he and I could live together "like brother and sister" while they stayed a couple. (Like an incestuous brother and sister, I guess, though she didn't know that!)
A couple of times I tried giving him an easy way out, asking him nicely if he wanted to break up with me (I didn't know about OWs and felt sorry for him because he was depressed and overworked) and saying that though I didn't want it, I'd accept his decision. You could see him struggling to make a decision, but he still said no, he didn't want to break up with me. But he didn't say that he wanted to stay with me, either, I realise with hindsight. His problem really was that he didn't want to do the breaking up. Plus he hates change and doesn't even like choosing a meal: he's a bad decision-maker. It took me snooping on his emails to resolve the matter!
My ex's mother had died months before, and that was why I had to be extra nice to him in our case. Your husband is using his dodgy mental health as protection. Maybe not in a deliberate, calculated way, but it's just easier for him if you "can't" treat him like a fully functioning, responsible adult.
Maybe it would be easier for you, too, if you could step out of the supposed responsibility for his potential suicide by developing your own mental health problem. Frankly, with the stress he is loading so generously onto you, it wouldn't be surprising if you became unstable yourself!
More likely you are just going to have to take on the role of the baddy, that he is so unwilling to take on himself. But it's not a choice between smiling and pretending, and angrily throwing him out of the house and potentially provoking suicide. There are ways in between. If you have arranged medical help, informed his family and made sure others are supporting him, and express deep regret as you arrange for one of you to leave, for instance, that already makes things a bit different.