Stepmom Help Please!!

I desperately need some advice so bear with me. I’ve been with my boyfriend for over 4 years. He has an 18 year old and an 8 year old who live with us 50% of the time and we have a 3 year old together. (All girls) I’ve always had a good relationship with the 8 year old and she’s a good kid but has been saucy to me a few times now. (Just the last month or so) she has a good relationship with my 3 year old but is extremely jealous when her Dad is around (before she was born she was jealous of her older sister) I ask her to tidy her room…clothes in the hamper and that’s about it. Tonight I saw a scribbler not realizing she was using it as a diary where she’s written things saying I’m a bitch twice (then crossed it out) saying I slammed a door in her face which never happened unless perhaps she came by the room when I was changing. When her Dad is working lately she wants to go back with her Mom and much I totally get but I’m starting to think it’s because of me. I have no idea where to go from here. Multiple times saying I yelled when I didn’t. PLEASE give me all of the advice