Joannof3, I think that your are in the middle of a war of control, and I don't mean this in a bad way, but there are too many complicated and different relantionships going around you.
First, I don't know of any child that doesn't want desperately their mother's attention, and also, your b/f is in the picture now.
One thing i see clear. You have to be the adult and the best protection for your children.
And Also i think your b/f should think more like an adult and put himself in your children's shoes sometimes.
Best of luck to you!