I may be misremembering or misunderstanding your overall sitch, but as best I recall, at one point, you were on the road to breaking up with this woman for many of the same reasons you point out now (her tantrums, unrealistic expectations) and then found out she was pregnant so you stuck it out.
I don't mean for this to sound harsh, but I hope people go back and read that part and see it as a cautionary tale.
I feel for you, Wolf. While I do agree that sitting her down to have an open and honest discussion where you validate (and make sure that you have your phones off so she has your undivided attention), I am not sure that is going to get you where you want to be because of her attitude. She's shown you time and again that she thinks your children don't matter. This has already caused you to lose your daughter and may cost you your son as well, not to even mention the baby you share with her. I also get the sense that she really doesn't care if you prioritize her child as long as you prioritize her. She seems to be pretty dead set on how you must treat her and how you have to do this and that and sacrifice. Of course, we are only hearing one side of the story and I'd bet her side is wildly different from hers.
My advice echoes Ginger's. GET A LAWYER! Prepare for the ultimate end. Yes, it is sad, but man, if you don't get out in front of all of this now, she's going to do a number on you and you'll never see that baby again.
While it is noble that you stayed with her for the sake of the baby, is it really worth it? You were already going to break up with her. This should be a lesson to anyone who reads it that getting pregnant never solves an issue.
Me 52, H53 Bomb drop 9/29/2014 Divorce from XH final 12/17/2014 Marriage #2 12/31/2019 5 adult (step)daughters (3 from XH's first marriage, 2 from current H's previous relationships) 6 grandkids