I would pull the headship thing and take your kids back. Life isn't fair, and you changed your mind. I'm sorry, but in my little world there would be NO issue about who decides if child will get blood. It's always going to be your decision.
I've been playing Ms. Nice guy for way too much of my life. I am near 50 now and I really regret bending over backwards all the time to accomodate others lives, feelings, needs while neglecting real needs of my own. Its good to do in some things, but bending over backwards all the time is BAD for you. Tends to make you "spineless". I'd say take back your kids, take back your life and if she wants to be the bad guy, let her. If she doesn't, maybe it will kindle something. Who knows?
I feel bad for you because I know what it feels like to know that you are the one who changed the game. I know how it feels to want to be with someone who cares about you and respects you.
My best to you. I want to get that little girl a christmas present and I hope she gets one next year.