I need help my 5 year old is out of control!!!

I call her my sour patch kid.. first she does awful things then wants to snuggle! LOL!! My little girl is so sweet and smart, loving, but she can be mean, rude and bossy!!! I can't help but think it is my fault some how... I am a single working mom and her dad is in her life but only every other weekend and he was away for about 6 months last year... she was always so good and respectful so I spoiled her not that I ment to but if she had been really good all day and we were at the store if she asked for a book or a small toy I would get it for her!
Well then she started school and I worked it out with my job so that I could be there for her after school for most of the week to help her adjustment to the new routine... so we bathed and picked out clothes the night before had a check list for things to do in the morning then when she got home it was a small snack then homework while I made dinner she could play or watch tv then it was eat bath book bed.... This was working great!
Till out of the blue my sweet baby went to school and this evil dictator came home in her place!!! Now before this if she got in trouble it was a time out (for both her and me) then we would sit down and talk about the problem and discuss ways we could both handle it better and what better choices we could make! I thought I was so luck my child loves reason and this was perfect I lucked out I would not have to worry about the spanking issue! I mean if my 2 year old could reason with me what would make me think my 5 year old could be so difficult!! LOL I was very very wrong!!!!!
My very smart daughter has learned how to terrorize me until I give in, give up, flip out, or want to run away from home!!! she is mouthy she will say something sassy then smirk at me! she throws the worst tantrums and no matter what I do I lose!!! I have tried going to my room to calm down when she gets me enraged but she fallows me continuing her tantrum I have told her to go to her room scream into a pillow have her tantrum then we will talk about it when she calms down I have tried time out taking away tv time, once when she was at school I took all her toys away and put them up and she could earn them back by doing what she was told with out complaint or helping me do some house work ect... she got home and thanked me for cleaning her room!!! I have even spanked her butt a few times which 1 hurt me to do it and 2 she turned to me and said that didn't hurt! so when I spanked her harder she told me she knows I didn't mean to spank her that hard! agh!!! I am at my wits end!!! I am stressed dealing with my own mental issues now I have this person bound and determined to make sure I have a total mental break down!!
I love my child but I am afraid that I may not be a good parent! I don't wanna mess her up!!! I am at a loss when it comes to her!!!