at this point, do you think I should go ahead and give her the ultimatium/confront her in a letter once I talk to the lawyer and have the okay? I'm really scared.
That is totally up to you, if you follow DB'ing then you will just ignore all that and continue to work on yourself. She's on her own journey and there's nothing you can do to help her, to speed things up or to change the outcome. But if for you the pregnancy (if she is pregnant) or continued A is a deal-breaker then proceed with the ultimatum. But the fact that you're scared makes me think you should give yourself some more time first, that's a decision you want to make with a cool head, uninfluenced by your emotions.
Regardless, I still think you should consult a lawyer to explore your options, find out if you're in a no-fault divorce state, and find out what (if anything) you should be documenting about your W's activities.
