Weary mom, I can tell you that it took many many months...more than a year...and very firm boundaries with a lot of consistency before my son stopped hounding me.
My husband says this about how long change takes: however long it took you to walk into the forest, it takes at least that same amount of time to walk out of the forest.
You and I taught our children that we will cave in If they just keep on. We taught them that no doesn't mean no.
Unteaching that lesson takes a long long time. Stand firm. Decide what you will do. Write it down. Print it out. Keep it close. Try hard not to react. It is going to take a long time to change. For you to change. For her to change. And she will ramp up like she did today when she doesn't get what she wants. My son has screamed that same word at me again and again.
They want what they Want when they want it. That is childlike behavior and childlike thinking. That does not work in the real world so they must change to survive.
The more we can stop, the more they have a chance to change. And the more peace we can start to have.