She's an addict. You really can't believe anything she's telling you. Maybe she was with him. Maybe she was feeding her drug addiction. Maybe both.
But the bottom line is, what you've been doing hasn't worked, and it may indeed be time to save yourself and your girls and let her hit the bottom. Some addicts need that in order to start to recover.
Get to some support for yourself - individual counseling or a divorce care group or Al Anon. And take the focus off of your hurt and abandonment (which I know triggers some childhood issues for you) and focus on your girls and making this as safe as you can for them.
Also - check out a book by Richo called When The Past Is Present - I think it might be helpful for your own issues.
((((hug)))) Sometimes it gets much easier once you drop the rope. "Let go or be dragged" was my motto.
