What to do with stonewalling

Hi all, I have posted here before but a bit of backstory: I had to move in with my dad and stepmom after abusive things happened with my ex boyfriend and bio mom. I have no choice but to live with them until I get fully saved up for a house of my own (renting isn’t an option, as I’ve done the math on renting vs paying mortgage, and am very grateful for their support in that aspect). However, my stepmom is the dictator of the house. My dad and I barely communicate as he just is not emotionally available and I’ve made my peace with it. I’ve tried to connect with my stepmom as she is the mother to my youngest sister. I just want to be close and I’m so confused on what I did wrong to make her hate me or despise me so much. I pick up on the subtleties of her just not liking me and see if physically and I’m just not sure why. I’ve tried communicating this with her and she swears up and down that she has no issues with me but it’s started to affect me mentally.

She goes through these periods were she’s so loving and supporting and apologizes for how she acts and then once my oldest younger sister (who is, obviously the favorite. This is vocalized a lot by both parents), is finally the favorite again, I’m thrown to the side.

I must say, too, that I have been away from them for 10 years. Spent most of my teenage years across the country and didn’t speak to them much because of things my mother has told me. Both sides have expressed things that have happened and I try not to take any sides but have no choice but to take theirs at this point.

I have changed everything I can possibly think of to fit their image and it’s not enough. I have no support system besides them. No friends. No boyfriend. It’s just me. Alone. And if stepmom is angry with me, and disappointed, the whole household is. But no one communicates to me what IM doing wrong so I can fix it.

I’ve done research on narc parents and she fits the category perfectly. I guess my question is, how do I heal from this while actively being in this situation? How do I learn to love myself again and not pine for some sort of connection with anyone? How do I do… any of this? I just feel crazy and invalidated in everything I do. It sucks that one moment I’m the golden child, and then the next I am the red headed stepchild. I just want to learn how to let go and focus on myself.

Thanks in advance for any advice