I really felt for you reading through this thread as 2 of my 5 children have behaved similarly. The youngest one was so difficult to deal with, but you could see underneath it all she was desperatly unhappy and I think she does feel that the world is against her.
When she was 7 she started talking about killing herself and I made an app with a psycholgist who wanted to see the whole family. We went a couple of times, but I knew that I couldn't go through with the prescribed treatment of not giving in to her.
She would sit at the top of the stairs banging herself off the rails or walls, screaming and crying.
She is 10 now and goes to a school which I think has heped her tremendously, and, as it's a boarding school, I can cope when she is home, (although I think the tantrums have stopped).
The last one was last year we had got to a caravan site it was late, we were trying to get the awning up, she was tired from the journey and she craves attention, which she 'couldn't' have right then. I couldn't cope with the screaming on a small quiet caravan site, I remembered one of my friends saying how she would hold on to her special needs little boy when he had a tantrum, I did this with her and it worked a treat.
Her back to my stomach, holding her hands crossed across her front and her legs trapped between mine, so she couldn't hurt anyone or herself, she did struggle A LOT, but I kept talking to her in a low voice telling, her I loved her and she was special et, etc, when she calmed down she was depseratley tired and sobbing, when we talked about it afterwards she said she really liked me holding onto her like that.
I always felt I was to blame for some of their behaviour, the 2 who were difficult were the most difficult births I had, and I had PND after both of them. I think middle child is also something to do with it, and they both have a desperate need for attention, which however much they get, is never enough.
The best bit of advice I got from the psycholgist was 'if you're going to give in, do it straight away', rather than, no, no, no ohh, go on then. Although it sounds like you're better at not giving in than I am.
Sorry for the long post I hope some of it helps.
I also saw this when I was looking for help on the net about my children and wondered if it applied.
What is an indigo Child?
As a summary, here are the ten attributes that best describe this new kind of child, the Indigo Child (named by those who predicted it).
They come into the world with a feeling of royalty (and often act like it)
They have a feeling of "deserving to be here," and are surprised when others don't share that.
Self-worth is not a big issue. They often tell the parents "who they are."
They have difficulty with absolute authority (authority without explanation or choice).
They simply will not do certain things; for example, waiting in line is difficult for them.
They get frustrated with systems that are ritually oriented and don't require creative thought.
They often see better ways of doing things, both at home and in school, which makes them seem like "system busters" (nonconforming to any system).
They seem antisocial unless they are with their own kind. If there are no others of like consciousness around them, they often turn inward, feeling like no other human understands them.
School is often extremely difficult for them socially.
They will not respond to "guilt" discipline ("Wait till your father gets home and finds out what you did").
They are not shy in letting you know what they need.
Indigo Child