Sorry to hear things aren't going well. With the long absence I assumed all the kinks were worked out. Again sorry.
To your question ..... when was it working well. Every thread or conversation you and I ever had he was against it ...he didn't sign up for it ...he doesn't want to share his wife with 2 other guys ....he tolerated it, to make you happy, to keep the family together, etc ,etc... forgive me if I'm wrong but I thought you said that once the kids were gone he would reavaluate the marriage at that time. The DADT, all those things are the definition of not being on board.
I think you have to give him some credit for trying. Unless it was just ways to run the clock.
If you didn't have kid how long would this situation have lasted? Or would it
have started at all... wouldn't he have said see ya later have a nice life?
Clearly he's mono lots of people are all over the world that's his choice.
You now identify as poly that's your choice or the hand you've been dealt.
Life too short for you to deny all that you are or compromise away the great majority of who you are or how you want to express yourself. And the same thing goes for him if he wants a full-time mono wife why settle ?
Youre both settling and both resentful. If you truly love him and want him to be happy you'd set him you free....and he 'd do the same for you. It's just incompatibility.....cramming someone in too small a shoe.