It's lovely to hear from you again but you sound so sad.
I very much hope that you and your husband are able to find a way to connect again. It always seems to me like both of you try very hard to be there for each other, that you care about each other and want to stay married. It must be frustrating for both of you to find you are unable to connect.
I wondered if maybe your relationship with C causes some of the distance from your husband? I think I'd find it very difficult to cope with being the 'room mate' while my partner was having lots of fun and good sex with somebody else. I suspect I'd feel resentful, hurt and angry and that those feelings would result in distance.
I know that ending things with C is no solution because then you feel the anger and resentment.
It makes me so sad that you both want to be together so much and yet being together seems to mean that one or both of you are dissatisfied and/or resentful.
I hope that in time you can regain the closeness that you have lost.