You are in a very tough spot HaWho. I faced a similar dilemma before my H pulled the trigger and served me with D papers.
The dilemma as I saw it was either leave the relationship because it was becoming increasingly difficult to live with someone so deep in the tunnel and consequently lose time with my S (who is 2 years old) or stay and suck it up b/c, hey, at least I get my S 100% of the time.
Perhaps look into your jurisdiction and find out exactly how much time you would be losing with your children. In my jurisdiction, when the child reaches 13, they are generally able to choose which parent they want to live with. If your jurisdiction has a similar set up maybe that can help you formulate a timeframe and a more detailed plan?
Sending you lots of hugs and support regardless of what you choose.
