At my wits end!

I don't know what I was thinking when I thought that after Nolan was born Chloe would shape up and start acting better. I was totally wrong!! It almost seems to be worse. Not sure if she just craves attention or just acts up because she can. We have tried everything to discipline her to no avail. Spanking her doesn't work cause she turns around and hits me or screams at her father. Taking toys away doesn't work cause she whines and cries and screams until the end. Time outs don't work cause she won't sit anywhere for any length of time. I so don't know what to do anymore. The girls always seem to want things when I'm feeding Nolan, so when I tell them they need to wait, what does Chloe do? Throws a tantrum cause she wants something right now.. AHHH!!!! I don't know any other child that acts the way mine does and it makes me feel like a horrible mother. I"m sure people think my children are cute and well behaved, but in the privacy of our home there is plenty of naughtiness and screaming. I pray that God will give us patience to learn how to deal with Chloe and that she will one day soon start to act better!!!
I"m open for suggestions on how to discipline!!!