Hello,
I am very sorry you have been suffering this situation for so long.
I have been in a very similar situation for the last eight years. My SD is now to be 24 and she has been cruel, mean, evil, spoilt, manipulative, and worse, boasting about it in front of her father without a boundary or correction.
She has stolen me things, broken things in our home and then made either a drama about it, or denied the whole thing.
I love my husband dearly, he is a kind, generous, intelligent and hard working man, however, I've pondered frequently divorce over this issue. He is totally blind and will defend her whatever she does because "his duty is to protect her".
This young woman was adopted when she was two days old by her parents when they were already separated after 20yrs of an eroded and childless marriage. Her father (my husband) left her mother when the girl was one year and a half old, but never meant to leave the girl, only the mother. Her Mum has convinced her that her father is the stupidest man on earth, unworthy of respect, and that leaks to everyone Dad chooses to be with.
I used to be like you, I'd suffer and mortify myself to a ridiculous point, I'd argue with my husband about his permissiveness an he would blame ME about the mean things his child would do (child at the time was, 16, 17, 18, 20, 21, till now 24yo). If she stole something from me (and newly bought good stuff at times) he'd say that I had too many clothes, or that I had made the garment disappear to incriminate his monster.
After a divorce threat and ultimatum I managed to make him go to therapy, and he attended for over one year. That has helped quite a bit, but there are still issues that are related to his guilt of having divorced his mum 22 years ago shortly after she was adopted, plus some sort of guilt-full pity he has about her for her being adopted, that will make him go blind about her meanness forever, plus the terror he is in about his ex after a very bad divorce with terrible fights about visit rights, money, and all the infamies this woman broadcast about my husband who is really a good, decent, honest, man.
My husband and I used to live in different countries separated by 3h ferry or 30min plane during the first one year and a half of our relationship. During that time, his ex would go about how his daughter was feeling displaced by me, even if I had to drive six hours, or 3h ferry, or 30min plane + Immigration to see my man and the girl lived (and still does) five blocks away from Dad's. The last seven and a half years of hour life we have been living in a third country due to my husband's job. If we hadn't moved countries, our relationship would not have succeeded to the point of being still together after eight years of attack (and yes, they can do it even if you live in another country, the creativity and energy they have for that is astonishing).
In my opinion, you have given her the winning hand at telling her how much hurt you feel due to her attitude. That is exactly what she wants and you have verbally confirmed her how cunning and successful she has been at that. You must look for no compassion or empathy from her (when my father died after a terrible disease in which I nursed him to his last breath, she stayed for two weeks with us after that and never had a comforting or sympathetic word for me, and she was almost 18. Both when my parents died her father would ring her in front of me, give her the news, and then pass me the phone for me to hear her to say nothing, otherwise, no sympathy would have been offered by her).
For your own good and all the empathy and identification I feel with you, just DO NOT SHOW HER SHE IS HURTING YOU and do not expect empathy from her, ever.
Everything she does to you, is because she knows beforehand that will have the desired result of hurting and creating turmoil between you and your partner because you show yourself hurt, and perhaps you ask help from someone who will never help and that is a father consumed by abandonment guilt.
You are confirming her how strong she is by showing and even stating that you are hurt. She does not care, period.
On the contrary, she believes that that is what you deserve and her father owes to her for having you at home and not her and her mum.
My SD has seized my husband by the hand from my side when walking on the street, made him walk away fast from me several times, and all that at 17yo. It has been at times hilarious to be disputed my husband by his daughter, but troublesome and marriage eroding all the same.
I could go over this for hours, as I am sure all of us who participate in this discussion could because these girls and their mothers in their emotional disruption/malfunction/disorder/perversity or whatever you wish to call it, can be incredibly creative, lying, inventing, defaming, blaming, manipulating, etc.
All I tell you is DO NOT SHOW HER YOUR WEAKNESSES, you have a perverse person in your life who wishes you ill, whom you did not choose to have and if you do not grow strong, she will destroy you and your marriage. She does to you what she does because she knows it will affect you negatively and generate issues between you and your partner. Do not please her in that.
I only wish I could help you more.
With my most empathetic hug,
Magdalena