I need real advice.

My relationship is falling apart due to my husbands daughter. And yes, she’s already an adult. A child but considered an adult. Her and I have never gotten along and I have always been very nice, cordial, loving even on her worse times. I have tried everything, I had taken everyone’s advice and I have even read books and online articles to get some advice on how to get a Long with an adult stepdaughter. Her main issue with me is my age, I’m much younger than both her mom and dad BUT still an adult. I totally understood this but I felt like even then it was no excuse to treat me like crap. It’s now been 4 years and I’m still dealing with her. Both her dad and I have spoken to her about her behavior. Her dad even sent her to her moms after she was out of line and didn’t care anymore. She came back, was still being rude and I was over it, I’m not having that in my own house anymore. She’s obsessed with her dad and everything that has to do with her dad she wants to be part of. I can’t even have alone time with out her being nosey even if she isn’t there. I have talked to my husband about how I feel and he has my back but also her back as that is his child. They have a a very close relationship, I know that both adore each other like I’ve never seen before. But she’s a little overboard with her love towards her father. To her, her father is like her king. I feel a little jealous sometimes because I feel left out. If it’s just him and her, she’s extremely happy. But as soon as I join, she even ruins my mood with hers. For example, tonight I came home and as soon as I laid with my husband she got up and left cause I was there. I even feel uncomfortable hugging or kissing my husband in front of her without her storming off like she’s mad at us. She even treats the dog better than our 2 year old. Yes, we have a 2 year old daughter and she treats her like shit. She’s still in high school (homeschooled) so she either stays home all day or goes to work with her dad and honestly I think she needs to find a job already since she’s 18. But my husband does not tell her anything. I’ve gotten to the point where her presence bothers me. I feel mad all the time and I’m taking it out on my husband. Him and I have been arguing like cats and dogs because I’m always mad. And I can’t tell him how I feel because that would really hurt him. I mean, I kind of already told him I don’t like her but he tries to always just have both of our backs because he knows how she is. I feel like I don’t even want to live here anymore. Not be una. Relationship with my husband. I am so over it.