I hate my DSD

Wrong. We have had massive issues having brought one child each into the relationship. Then having further children and both our older children having much less than ideal blended families in their other parental relationship.

In fact we also had SS involvement due the meltdown of one of the teens in the family who went of the rails.

Why? Because they had a large amount of very complicated and emotionally challenging situations happen in their childhood that that their siblings and step sibling didn't while trying to swap and fit into two very different families on a regular basis.

But DH & I have enough emotional maturity to understand why and how it happened. We also clearly understood how the decisions we made while for our benefit and also ultimately our kids together definitely didnt feel like it was for there's even if as an adult they feel like it has overall been.

The last part means what support did your DH ask them for?

Why did you get involved with a man with baggage and even had kids with him?
He was someone's daddy, why start another family with him? Was it money or what?
And what did you expect?

Sounds like your DH is not a very good parent. Has he just allowed her behaviour? If so I guess the kids you have with him will turn out to be nightmare teenagers too.

You're not being unreasonable, I was a horrible teenager and if I had a child in my life now that was similar I'd feel the same as you. If you want to stay with her dad you've just got to ride out the time, maybe find a hobby that gets you out of the house for some of the day. She will probably be nicer/more bearable when she's a bit older.

OP if she has lived with you for 2/3 years - when did the behaviour start? What are the allegations- you say neglect but in what way? What happened that she needed to live with you full time?

Honestly, really think about leaving, you'd potentially have split custody and where your children could be left alone with her. Can she go and live back with her mother and see her dad out of the house?

How old was she when her parents split up and her father moved out? You have a 6 year old, so SD was 9 when that child was born? then 2 years later along came your second child. At 12 she left her mothers home to move in with you, her dad and your 2 young children. So in her eyes first she was abandoned by her father, then her mother, then she has to adapt to having two very young siblings who will take all of her father's time. You may have tried your best but for some kids thats just too much. She and her dad probably need to go to therapy together, its not just her problem to fix.

This. Leave.
Life is too short to put up with awful kids.

Did your husband ask the social worker for help with his daughter?

There’s literally no ‘reason’ for this age. Look at all the things we protect children from
until they’re older - from marriage to caffeinated drinks.

Eh? Are you really inferring that the OP will harm
her step child?

Don’t be with her parent as they should always put their child before their partner.

It's worth remembering that various step-parents who have killed their step children have been known to previously tell others that step child was 'bad', 'evil', etc.

Maybe those that read a few lines from an anonymous poster on the internet can consider that before they decide it's okay to call an unknown child "vile" and "a little shit"

This sounds horrendous OP.

At 15 you know right from wrong and she sounds like she can play any system to her advantage.

id hate to advise you to end your marriage but I think in your position is be considering it.

Where are you from? The UK has quite low rates of step families compared to loads of other countries like US, Canada, France, Germany and Sweden.

OP, 15 year olds are little shits. We don’t kick them out when they’re our own because we love them, even though they’re unbearable. I think it’s really difficult for Step Mums who have no prior experience of teens. I can understand that it’s awful but believe me, one day you’ll be tearing your hair out with your own little ones.

We have stayed married yes. I left them to it for years. Literally did nothing, mostly had plans and did my own thing when she came - she told me at 17 she hated me so that was that. So me checking out suited everyone at the time - I’ll be crucified for it now though as much as if I’d been involved.
our marriage is a happy one and I am glad I stuck it out but I didn’t have young children and none together, my kids were teenagers so easier to be apart a few weekends a month.

I hope you can find a way through for all of you

What were the issues? It must have been quite serious given that she moved in with you when your DC were, what 1 and 3? How were things at that time?

Fully get the need to vent but you need to also find a way through it, for your own sake and also for hers and your small children who are growing up in a toxic environment.

I hear you OP.
Step parenting is a thankless task and until you've been one and had to deal with all the shit that comes with it, you'll never, ever understand. So many second marriages break up because of the conflict from the stepchildren, ex wife/husband, the torn mum/dad trying to make everyone happy. Its a shitstorm at best.
Its like people who think fostering will be taking on some poor, neglected child that will be so grateful for your warm, loving home and look at you with big doe eyes, forever in your debt for rescuing them....

Im a SP before you start your pity party of no one understands.

You are comparing apples and oranges in your children, your DH parenting of them and her. Your children have not and hopefully will not have the same lived experience.

She isnt psychotic or a lunatic. She is doing what all teenagers who struggle with self image, attachment, poor parental relationships etc etc etc do. She is seeking validation in all of the wrong ways.

Hate her all you want. Praise your husband all you want. The reality is he is 50% responsible for the childhood experience she had has. From choosing to have her with a woman you clearly have so much distain for. Then even though her mother is apparently shit, was happy it seems for her to only move in with you 3 years ago all the while having two more kids.

Teens who go off the rails have being struggling far before they do.

And so you ended up with SS involvement. You arent the first you wont be the last and you say she said she was being neglected. In her mind she probably is.

They came, they saw they left but tell me what did your DH seek from them for support before they did?

OP you describe her as psychotic- is she being treated for this? Antipsychotics are surely the first line of treatment if she's experiencing psychosis; is there a family history of schizophrenia or other psychotic disorders?

Stepparenting is incredibly hard because you do not have the unconditional love for a step that you have for your own. You do not have that thing where you love the bones of them and even their most awful behaviour is possible to forgive and even paint in a good light. That’s why stepmothers from time immemorial get a bad rap. Taking yourself away from the situation is the correct response.

It says troubled and hurt.
Why is that?
Look at her childhood ?