At this point, it seems to need to protect yourself and your son. Decide what YOU want in a divorce, talk to a lawyer and proceed, otherwise you'll get caught like a deer in the headlights.
The slimmest chance your ex will ever turn around is if you take a hard line now. But then, can you really trust her? She seems to be serial cheater.
I suggest you go the Chump Lady website for some humorous but useful perspective.
