Today A big warm welcome to you. This forum has helped me a lot and I hope it helps you. Your story made me sad, and I could sense your feelings of isolation.
When you married your husband he was in the world..so it's him thats changed the goalposts, not you. It's him who now expects you to live in 'half' a marriage while he's married to the Org mob. And he should be so thankful that you love his daughter as you do, all credit to you.
If you want to fight for the marriage I'd follow the advice of a previous poster who said in essence - Make his life at home, (and especially ) before and after meetings and ministry times so loving and enjoyable that he will begin to notice the difference between the 'conditional' love he gets at the Org and the genuine love you feel. Dont criticise the Org or say anything to antagonise him about it. Just smile sweetly and ignore all the rubbish.
If you dont want to fight for it well separation and divorce is painful so be sure it's what you really want?
Most of all, do not let your son get involved with it at all. Your instincts are correct on that score, and he sounds like he has his head screwed on.
As you continue to deal with the distress of managing all this - come and talk to us on here and get some relief and support for yourself, its a good place to be.
Loz