An apology for what? That you dont like her crap sandwiches anymore and you stick to your boundaries. You want to move forward. All of those things SD doesnt want. If she is anything like mine, and sounds like she is, she wants to stay stuck in the toxic drama. They thrive on this. 

Keep being you, and kick her out of your head. She just isnt worth it.

Never give that B an apology. You didnt do anything. Frecking pain in the butt these toxic SD's

Let me guess DH backs the lil B up and you are the bad guy. Thats how it was for me.

I gave BM, now dead, and still give SD59, way too much of my brainspace. It's probably natural, when so much of our emotional energy has been expended, to be on hyper- alert and thinking constantly about these toxic people.  The only thing that has helped is gray-rocking SD59.  I purposely dont bring her up to DH, when he talks about her, I go "Hmmm" or "Gee" then change the subject.  I do have to see her once in awhile and i'm civil and polite but thats all.  I dont initiate any discussions with her.  When she starts talking about something, I dont make any comments other than " Hmmm" or "Gee".  

I don't know if this will work in your situation but it has helped me stop thinking about her so much.  I realize she will be in my life as long as DH is alive and I realize I will have to be involved somewhat since she's on disability and we help subsidize her.  But limiting my interaction with SD59 has been very beneficial to me.

Good luck.

If you apologize she will take that as you admiting to whatever her twisted mind has decided is the "true" reality. 

I would tell her I'll stick on the horn of the pink unicorn it will fly in on.

She wants a clean slate so she can re-start the drama again.  When you're disengaged she can't and its driving her mad.  Keep disengaged and tell your DH you're not apologizing so stop hoping for it.

Agree with JRI, grey-rocking is awesome!

What horsepuckey! While there are times I have apologized for the sole purpose of moving past something (ie: I'm sorry we've been in disagreement"), this is NOT one of those situations. She WILL take that as you admitting to whatever her twisted mind has decided is the "true" reality.

Definintely go grey rock and carry on with your life. 

What'll she do if you don't apologize? Not speak to you? Great! 

I feel your post so much.  I have been married 9 years, and have my own son 22, SS21 who I have always gotten along with and for the most part is a respectful person (although has suffered some damage in this divorced situation), and SD almost 18.  SD and I were good in the beginning. That started changing as she became a teenager, she stated acting out with entitled attitude and flat out rude, and my DH catered to it.  It got to the point where I was pretty depressed, and I withdrew a lot.  She and I did not interact much for a while, and DH and I were not getting along well, mostly due to his lenient parenting.  The nail in the coffin was when I asked him to have her start letting us know when she needed to drop in when she was with her mom, she had taken to coming in at random times sometimes more than once a day, and it drove me crazy.  She lost it on him and said she felt "unwelcome".  I just wanted her to be considerate and thought she was being ridiculous, so it just made me more resentful as she kept testing that, and of course she was mad about it as well.  Not good for our relationship.

After a rough year or 2, DH and I went to counseling and got more on the same page, and that helped me out of the fog of depression.   SD was acting out at times, not always coming when she was supposed to and withholding communication from DH whenever something upset her.  I did feel responsible for my contribution to the situation, I knew I could have handled things a lot better.   I ended up approaching her, hoping to open the door to talk by letting her know I love her and I also apologized for ever making her feel like I didn't want her here.    I of course ended up crying, and she just looked at me like a deer in headlights.  She did give me a hug, but my hope that it would open the door to clear the air and move forward didn't pan out.  She didn't really say anything.  

I don't know if it meant anything to her, but I also don't feel like I was giving her something that made her feel more powerful.  Granted, she is still a child and your SD is an adult, but in my case I genuinely never wanted her to feel like I didn't want her here, and I am glad that I apologized for that.  In my situation, it was the right thing to do, and I felt lighter.  I did not and will not apologize for my expectations in my home though.  Unfortunately they are not something she feels she can live with because she has not lived here in 2 years, or maybe there are other reasons.  She has demanded apologies from DH, but had never been able to tell him for what.  Just that "he should know".  He is not sure what to do with that, the only thing that comes to his mind is apologizing for not raising her right since she acts the hateful and entitled way she does.   But that won't go over real well.  

I have felt anxiety regarding her for a long time, and given her tons of space, energy, guilt, and tears, but it is a little less now.  I feel like I have made an effort to move forward, and that is all I can control.  SD has shown herself even more recently, and I can see that while maybe I contributed to some things, I am not the cause.   A lot of my anxiety came from guilt.  Now I just feel anxiety because I know that if I do or say the wrong thing, she will use it as an excuse to attack DH.    I haven't seen her since  Christmas, which was a mess due to that very reason, and she is mostly not communicating with DH unless she needs something.   But with her graduation plus our boys from college, I know I will be seeing her before long.   The best way I have dealt with it is praying a lot!  And also as JRI stated - gray rock when I do see her.  Polite but reserved.  Which that can when be nerve wracking because then I might seem unwelcoming!  It an exhausting cycle that I don't think we can win, but I also don't think we are really the true  focus of their drama, we are just a convenient target.   That take some of the pressure off for me, in a strange way.      So sorry you are going through this, I will say a little prayer for you too!