Wow...I've had similiar experiences so close to share....luckily for me they are gone now but I am so sorry you still have to deal with a ***** like that.
My past step mother was the same thing with me when I was a little girl, the 5th through 8th grade I believe. It was like a literal Cinderella relationship...add a shy, homework-hating, dreamy girl like me and a pompous step brother and it would be complete. I was expected to do a lot of work, not that others didn't share the same effort and not that I'm complaining about having chores, work is a good thing...BUT if I messed up one little thing, did not cut the grass to the precise centimeter, missed one spot on a dish, didn't dust one shelf well enough...I was expected to go back and do it all again after I was yelled at and called lazy and worthless. Meanwhile, her son, who had the same chores, always seemed to get them perfect....he was never yelled at...When he would ask to play video games it was yes...when I did, it was no. It got to the point where I was so nervous to ask if I could do anything that I thought about just how to ask her for hours and sometimes chickened out of asking something so simple like: 'can I play a video game?'
Her son, Cody, was the favorite. He was so well liked and got so many things he wanted. Didn't get in trouble like I did. If his friends called, he could talk to them for hours. If my friend called, I was yelled at: 'your friends shouldn't be calling the house! tell us before your friends call!'
All my talents were crushed. She was always asking me why I wrote stories and when I told her 'for fun,' she accused me of rolling my eyes, literally kicked me, and said 'why do you even do that? all you ever do is steal ideas from other people! these stories can never be published'....so she was also a snoop. She got into all of my notebooks and read all my stories and diaries without asking. And that wasn't the only spying she would do. She would often poke her head in my room then leave without a word.
Once I was doing bad in a class because I was too depressed to do my work, and my punishment was very similar to the one you described: I was locked in my room for weeks. After I got home from school I went to my room and the door was closed. No phone, nothing. All I had was my homework, my stories and art, and a radio. I was not allowed to eat or do things with the family. Just stay locked in the room. I could leave only to go to the bathroom and to eat a seperate meal from everyone else under her careful watch. This went on for weeks, even on weekends I was locked in my room ALL DAY, going crazy. My room was in the basement, so I had no way to sneak out cause she'd catch me.
Once I came home from a friends house with make-up on....I was so happy because everyone at school always called me ugly and finally someone offered to give a make over....She ordered me to the basement, to my room, ripped off my glasses and threw them, grabbed my face and smeared the make up all over till my face was black, pulled my hair (she noticed it was straight now) till it hurt and commanded me to go and clean it all off, now. A plate of dinner was thrown at me by my father who was mad that I had to come home early (he hated driving to pick me up), then I was sent to my room for the next day.
She made me so scared that I WANTED to stay in my room to get away from her mean comments and stares.
She made me so scared that I TRIED COMMITTING SUICIDE while at my grandmothers in the hope that I would never have to go back to her house.
My dad finally saw how she was treating me, and him one morning, saw the light. They had an argument about whether I was supposed to eat with the family or not, and she ended up slapping him. He said his spouse could never hit him and it was the last straw. We packed up and left.
For a year after, they dated. She scared him because she seemed to be spying on us, what we were doing, where we were going, what e-mail he had and the women he was talking to. Once we came home to see that his stuff had been gone through...she had a key...we thought maybe she broke in and went through his stuff....
Anyway, it is over now. I still have to see her everyday because she works at my school but it is okay. He has a new, wonderfully sweet gf that I love and to my knowledge the old wicked step mother doesn't stalk us anymore.
But I look at you and see you still have to suffer. I am so sorry and wish you do what you think your heart says is right. *gives you hug*