Dd is 8, beautiful, funny, and very bright, but she has always been difficult. As a baby she didn't take naps and was very clingy ie she would scream if I left the room, she would cry if anyone else held her including her dad and grandparents. As a toddler the word no didn't seem to register, if she wanted something she would scream and tantrum, I ignored bad behaviour and praised good, but even when I ignored her persistent screeching she would kick hit and just get louder. 
She was an early talker, we could have a conversation at 18mths she did well at pre school and when she started school she excelled in everything, school reports are amazing, teachers say she is a leader and is respected by class mates, she's supportive of others and has lots of friends, me and her dad are very proud of her, BUT at home it's a completely different story, she's whiny, demanding, defiant, she winds her 10 year old brother up till he's in tears, she screams, shouts, bangs and slams around the house and quite honestly we've all had enough of her attention seeking, drama queen behaviour. No punishments seem to work, we've taken away toys, stopped her from playing out, even banned her from Brownies for a week which she hated us for and screamed for the entire hour and a half she would have been there.
Bedtimes are a nightmare taking at least half an hour of messing around before lying down.
I'm at the end of my teather, her brother is the complete opposite, quiet, laid back, easy to please and sensitive to other people's feelings, I feel like because he's so laid back he fades into the background because me and dh spend so much time dealing with dd, I feel sad for him too.
What am I doing wrong? I'm certainly not coping well and question my parenting daily, I'm failing both of my children.
Sorry for the long post, if really appreciate some feedback.