Feeling pretty pathetic but?

i feel backed into a corner. im not sure what im meant to do. im a 23 year old stepmom of a 13 year old girl. if She asks if she can do something and i dont agree with it i ending up with my husband angry at me as she moans to him. i feel like i cant do anything right. like im meant to let her get away with everything she wants to do just so i get some peace. can someone please help me with some advice. I have no one to talk to about it. i feel so down and i no its stupid but there is literally nothing i can think of to do.