My situation came from where you are now OP but, is slightly different in that I don't see the perpetrator of my unhappiness. She's 20 yrs old now (have known her since age 11) and has refused contact with me (and pretty much her dad too, unless it's on her terms) for 3 yrs. Her nastiness has tainted any relationship I might have had with my eldest SD (25) and we used to get along well; she liked me and I had a genuine affection and interest in her. Now, even with eldest SD, it's horribly strained and tbh, I feel physically wrecked before even she arrives, which isn't that often. Youngest SD, right from the beginning, would ignore me (she was only young, I know) but it wasn't addressed and continued. She wouldn't speak to me/look at me/acknowledge me. If I spoke to her, she'd reply to her dad (it was like I wasn't even in the room....like she was answering an echo!). Dad (DH) did nothing. In 2011, I told her to clean her room, she objected and I haven't seen her since. Apparently, I had no right to tell her to clean her room and so, she's been punishing me and her dad, since. Obviously, it's not about the room tidying. It's about my coming here in the first place, marrying her dad and usurping her position as First Lady, in dad's life. I'm NOT the OW. Dad was long divorced when we met, as was I.
DH apologises for his kids. It has caused massive problems in our once rock solid relationship. My own ds (whom DH has known & loved since age 4 (youngest SD resented that too) has had a dreadful time, because of it). I detest my youngest SD and the situation she has put us in.
I feel physically ill at the thought of any contact with her now. I don't mean I just don't want to see her I mean I have an actual physical reaction, near-panic attack.....pulse goes up, breathing hard, massive anxiety. It's madness.
No advice really just....you're NOT alone.