Stepmom of 5 years with anger problems

From the beginning I had attempted different forms of punishment from time out - taking things then of coarse a spanking 

She skipped the "terrible 2" and hit " terrible 3 and never left them. her tantrums have become worse - once in Walmart she threw a fit because her cousin was with us and my stepdaughter picked out some cereal and my step- niece said she didn't like it so I said let's look before we decide and then my niece decided that she did want it -I think it was more about who got to put it in the buggy and then I didn't get the ice cream my stepdaughter had seen at the front of the icecream isle I told her the we were going to keep looking to see if there was an ice cream that both her and her cousin would like she immediately cried and threw her fit I told her if she could calm down that I was trying to make both girls happy with one choice of ice cream- I was not going to give in and just get her what ever she wants - but she didn't calm down so we went toward the check out isles her fit got louder so I then left the buggy at the check out and went to the car - her fit got so much worse - she crawled back to the buggy I tried to carry her and she wiggled and screamed and kicked her shoes off - everyone was looking. her cousin was embarrassed - we got toward the hair salon and she picked the bench up and slammed it up and down several times - I tried not to give in the her fit - I wanted to spank her right there but everyone was watching and staring - we finally made it to the car and she was crying and screaming " I don't want to get a whipping " over and over - she got a whipping at the car and she kicked and screamed louder and when we got home she got another one and I made her go to her bedroom where she still screamed this lasted about an hour and a half of continuous screaming and crying alone in her bedroom - I called my nieces mom and asked her to pick her up because my stepdaughter was misbehaving 

she did it at redbox when we just clicked rent movie I had seen one on the first screen that me and her dad might want to watch well she threw a fit cause she didn't get to pick firsts tried to explain that I had seen one that we liked and she was going to get to pick out a movie she cried and stomped her feet and another time she was pushing her cousin to was her hands when her cousin was already there I told her to wait until she was finished and she got a huge attitude fell in the floor and yelled at me -  she has no patience and I have tried several things to teach her this --- I don't know what else to do I had to call her dad the last time - she starts school this Monday and her mom and dad have split custody usually Friday to Friday but because we live in Blounrville and she lives in Church hill we talked about putting her in school here so now she will be at our house a lot more - and I'm not sure I can handle this -I feel sorry for the kid because her mom doesn't care for her like she cares for her mom- the only complaint for us putting her in school was she didn't want to pay child support - I think she would be happier with her mom but she is way better off with us because in the 4 years we have been together she has liver in 10 or more places and has had about the same number of cell numbers - my husband has tried to call her several times this week and her phone has been disconnected - I need help on how to deal with her  HELP ME PLEASE!