Oh S.! You and I could talk friend! :)
I will start by saying my ex left us when my daughter was 4, he moved 1,000 miles away. So I thought all was well we were all adjusting, then my daughter turned 5! I literally was in tears with her daily. Part of it was anger and confusion I found out but some of it was just good old being a five year old girl. I literally called up the Pediatrician one day in tears saying I was failing, that I didn't know what to do or how I was screwing up but I was tired of spending every day arguing or disciplining her and like you NOTHING was working.
We went to a play therapist, that helped us big time, we got past the hump of questions unanswered with the divorce, how to communicate, how to be more understanding of each other and really opened up a world for us.
I fought with my mom from 11 to 21 and I did not want that for she and I but I knew if something didn't change we were already headed that way!
So, first, there is something that matters to her greatly, for my daughter, it was bedtime. So when she acted up, every sass it was 15 minutes, for big defiance or hitting her little brother 30 mintutes off. I stuck to my guns, one day she was in bed at 2pm! NO JOKE. Now she didn't sleep and came down for dinner and went back to her room but no matter how much she begged or cried, I was TOUGH and she had her punishment. That was hard, I like you have not always been consistent but a few times of her really understanding I meant what I said was all it took.
Then came to putting choices back on her. You can have a good day or bad day, but you are in control of yourself is what I told her. I told her to practice every day of thinking before something came out of her mouth so she didn't get in bigger trouble, took a lot of work. Then I gave her one on one time with me, all her and me. Her brother went to bed 45 minutes before she did and she and I played, read, drew, laughed, snuggled and she got my undivided attention. That was HUGE for her! Once a child feels they are always getting into trouble, they see little point of why to be good. THAT CAME FROM MY DAUGHTER by the way and is very valid.
Try to decide too is it worth it? Pick your battles, give her some contorl over herself and when she makes bad decisions don't yell but look at her and say "well you choose this for yourself, I am very sorry". It takes time to sink in she is what is causing her problems, not you.
Be careful how often you lose your temper ( I know how hard that is after sassing and slammed door and defiance believe me!!!) but take a deep breathe, walk away. Once she saw me practicing calmly down before I addressed the issue she started calming down. BE CONSISTENT, mean what you say and say what you mean. There is something big to her, computer time, TV watching, bedtimes, something that matters enough to use that as your bargaining chip on her bad decisions.
Praise her too, remind her how much you love her daily, praise every good thing she does. Kids are all different and while my son doesn't need as many pats on the back I see my daughter really thrives on positive reenforcement!
My son just wants some snuggle time with me and my daughter really wants me to look her in the face and hear her!
Letting them know you hear them, understand them and that you want only what is best for them helps tremendously. It will not happen overnight, it is work but well worth it.
My daughter still has the drama mamma moments that test my nerves but I can look at her and say "is this what you want for yourself, because when you have to go to bed early I miss my one on one time with you"...so she doesn't see me as this bad guy wanting to punish that I have to be a good mom and teach her what is right and wrong.
I tell my kids every day it is much easier NOT to discipline and let them run crazy and not care, but being a good mom is my laying down rules and them abiding by them, to give them manners, respect, compassion and responsibility so when they leave my house one day they are prepared to be great adults! My daughter has told me she is going to be a strict parent when she gets bigger so her kids are good people! Not sure if that is bad or good but I know she hears me.
Good luck, deep breaths, don't worry about your 2 year old picking up on her behavior, he will be watching your reaction and believe me when he sees you lay down the law, follow through he will know where his limits are. My son is four and has watched his sister lose priviledges, go to bed early and he is a lot better for it!!!!! Just stand tough, but with gentle guidance, love and support and communicate with her. HUGS