why why why?

I do realize that parenting is not all daisy's and roses. I do realize that there will be trying times. I do realize that this will be worth the struggle. I just don't know how to make it through this one. I know I can't quit... but I just feel so frustrated. She won't bend. She won't learn from the same mistake. She won't pull it together and do the right thing for a change. WHY WHY WHY??? What makes a person abnormally stubborn?? I know, I know... "this too shall pass..." I just want to know.... who took my adoreable, smiling, sweet tempered little girl, and left this horrid, lying, sneaky one as a replacement??? I miss the girl I used to know.