I feel for you. Having a mental issues compounds already difficult situations. I struggled with bi-polar disorder for years and was "high functioning" most of the time, but over a fifteen year period I would flip off the scale every 2-3 years. It definitely impacted the way I felt in my marriage. Pretty powerless really, because I felt (and my ex-husband reinforced) that I wouldn't cope without him. With that experience in mind I can see that perhaps you think, and/or he thinks that you wouldn't cope with him taking on another partner.
In my new relationship I also cried a lot and really struggled when I realised that my partner was actually "in love" with his very good friend and she wasn't just going to fade into the back ground as a result of our relationship. He tried to give her up but he wasn't the same person. It was like part of him was missing, or I could feel his sadness coming between us. Anyway In the end I gave their relationship my blessing and started actively embracing the idea of polyamory.
What I have learned from all this and what could probably be applied in your situation is that for people like us especially, it is best to move forward in baby steps. For me that means tackling my seemingly biggest challenge from its smallest practical point. In your case that sounds like you need an explanation from your husband as to why his need for another came up and honest communication about your ongoing feelings. And I think that is entirely appropriate.
Does he understand your condition? If he does then he should understand that something like this will not go away it will fester and needs to be talked about.
Hope that wasn't too much of a ramble and was helpful.