I know you're having a really tough time of this...so here's the last I'm going to say to day (cause I have to go for awhile).
He married YOU. You have the right to ask if he wants this marriage to work. If he does (and I know you dread him saying no to you too.) want this marriage to work...then the EA MUST end.
Trust is a 2-way street. He cannot expect you to prove yourself to him while he's cheating on you...that is not only unfair, it's cruel to the nth degree.
It's as if he's saying to you...if you don't do things just right, he has a backup plan. Don't let him use you that way.
I'll throw this out there too..and I may get blasted for it. But if you really want to fight for him...then call her on the carpet. You know how to reach her, confront her. Sure it may piss him off...then again, he may see you fighting for him too.