Have namechanged for this as I feel so ashamed.
DD is 12 and has been 'difficult' for about three years now. When I say 'difficult', I mean lazy, obstructive, aggressive and manipulative. That's when she isn't getting what she wants. When everything's going her way she's actually quite charming and funny, but as she grows older and we expect more of her, opportunities for conflict are multiplying to the point where family life is becoming really quite seriously unpleasant on most days, and the stress is taking its toll on my mental health and my marriage, and also on the health and happiness of my other two dc's.
The situation now is this. She won't voluntarily:
Do schoolwork
Go to bed when we tell her
Get up in the morning for school
Clear her plate from the table
Help with any housework or chores
Hang her coat up/put her uniform and school bag anywhere except in a crumpled heap on the floor
Tidy her room. All her clothes/books/bits and pieces, are in piles on the floor and under the bed
Or actually do anything we ask her to do if it doesn't suit her, without repeated requests, which are then met by rudeness, huffing and accusations of 'nagging'.
So far this probably sounds fairly unpleasant but typical 'tween' behaviour. I could tolerate it if this is all it was, even though it's exhausting and demoralising to have to keep telling a child to do things that they should be doing for themselves.
No - what's really destroying me is the fact that requests for her to help/get out of bed/go to bed/tidy her room, etc, are routinely met by indifference, stalling, rudeness and, eventually, aggression.
She also manipulates and stirs my younger 2 dc's up (ds's are 6 and 8) so that I have great difficulty managing their behaviour as well.
My youngest ds has aspergers and can be very difficult. He's tactless and can be obsessive and hard to divert when there is something he feel he needs to have or do. Obviously his behaviour can be hard to manage and I sympathise with dd for having to share our attention with a sibling who needs a lot of parental supervision, and tolerance from the whole family. But that is the way it is and we can't do anything about it other than try to accommodate his personality. DD refuses to make any allowances for his special needs. If he says something stupid or rude to her she pounces on it instead of ignoring it, arguing with him to the point where he'll explode - and over nothing most of the time. If she's bored in the car she'll subtly wind him up until they end up hitting each other. Almost EVERY journey with them is torture for me at the moment because of this, and I'm scared I'll end up crashing the car one day with them fighting on the back seat.
She constantly accuses me of favouring him and treating him differently from her and my middle dc, of not punishing him enough. Well - I do try to address his behaviour if he's aggressive or rude, but I can't pick him up on EVERYTHING because it simply isn't possible to do this and get from one end of the day to another holding body and soul together. I've explained this to her, and explained to her that he has difficulty with controlling his impulses, and also that he isn't able to read people's feelings in the same way that she or the rest of us do, and that as a family we have to try to find a way of dealing with this. Her response is to say that everything is my fault, that she has a friend who has 2 brothers with autism who manages much better than me. It's because I'm a 'crap mother' apparently, and 'everyone says so'. 
She's physically quite aggressive with the boys, although I never see her do it. She always insists that it's an accident - her hand or foot slipped, but I know she's slapped them around the face or head on many, many occasions. She also pulls faces and sniggers at them when I'm telling her or them off, so that my attempts at discipline are done against a background of their laughter. This is a new thing - and when she did it this week I lost my temper and told her that the way she was behaving me me feel so angry that I felt like picking her dinner plate up and cracking it over her head. Obviously that was a stupid and totally out of order thing to say, and I'd never do it. But she took that comment and ran with it. She overturned her plate on the table cloth and walked off screaming abuse. She shouted at me that how dare I threaten to break a plate over her head. When I tried to point out that I hadn't threatened to do this, but that she had made me feel like I wanted to do it she stuck her fingers in her ears and shouted 'shut up, shut up', only taking them out of her ears to shout more abuse at me. I managed to calm the situation down enough to get everyone ready for bed, then went off to work after DH came back.
Last night another horrible scene. She marched over to me and told me to get her blazer out of the washing machine and dry it. I told her I was busy and that she had to do it. After a five minute argument she went downstairs and roughly pulled it out of the machine, along with half the rest of the clean wet washing, which she left lying on the kitchen floor. I politely asked her to pick the washing up and put it back in the machine. Her response was 'no. Are you going to try to make me then? If you touch me I'll call the police'. I repeated calmly that she should put the washing back in the machine. She then came right up to me, invading my personal space and stared in my face with her arms crossed - said she wasn't going to do it.
I walked away as it was clear to me that we'd crossed a line and that I didn't have anywhere to turn. Told DH who (again - calmly) went in and told her to go to bed straight away. It was about 8.30pm by this point. She took her wet blazer upstairs into the office room where we keep the dryer, but instead of then going to bed she just stood in my office, touching one of the tools I use for work. I told her to go to bed and to put my tool down. She ignored me and went to walk out with it. I asked her politely and firmly to give it to me, and eventually just took it out of her hands with a bit of a struggle, as she refused to let go of it. She walked ahead of me down the hallway, saying 'why are you following me' (I wasn't, I was going back downstairs), then as she got to the end of the hallway kicked a bin which was standing there waiting to be emptied. The contents went all over the floor. I told her to pick the rubbish up and put it back in the bin, and once again she outright refused, just smirking at me with her arms crossed.
At which point I completely lost the plot, grabbed her pyjama top, pulled her down towards the rubbish and bellowed 'PICK IT UP!', about 5 times. When she pulled away and shouted back at me, I pushed the bin and the rubbish into her room and said that if she didn't want to pick it up, she could have it on her floor so that the rest of us didn't have to step over the mess.
I was shaking all over and mortified at having lost my temper. DH came up from downstairs, calmly made her go into her room and I went off and sat in the kitchen with a cup of tea and 15 slices of toast. This morning she blanked me and deliberately went off to school without her packed lunch.
I'm shitting myself for her coming back from school. I found her behaviour last night intimidating and horrible and I just don't know what to do.
I'm worn out with all the arguing and the conflict. And over - what?
Our family life should be great - we have lovely extended family, a decent home, enough money coming in, our health and each other. DH and I are ok - we don't argue, but the stress of managing our children's behaviour IS a strain on our marriage because we are both so tired and sad about it.
And last night I had this thought: "only another 6 years to go and then she can move out". That's just so wrong. We should be enjoying our children, not constantly, constantly battling with them over every day life.
And the worst of it is that I'm blaming myself all the time - I feel so inadequate as a parent. And I think everyone else blames me as well. I feel like I've not been tough enough with them in the past, that I haven't instilled good habits and now they're going to grow up lazy, rude and helpless.
And the guilt is making me resent dd SO much.
Sorry for epic post by the way. Well done to anyone who's got this far.