I remember DD at 6. She also became incredibly defiant and generally made life miserable at home. It was the second time (first was 3.5 years) that I went back to reading parenting books wondering "who is this child?" I recommend "Your Six-Year-Old: Loving and Defiant" by Louise Bates Ames. The series is old, but has such great advice. This is in addition to all the advice already offered. Sometimes it may be a developmental issue that, thankfully, will resolve. Hang in there!