I Want to Run Away Because Nobody Cares About Me

So we've come to this, have we? After too many late nights, a non-sleepover birthday party, and too hot to sleep nights, Kenzie has morphed. She became very saucy and was talking back, telling me what to do and being nasty. Who was this child and what did she do with my sweet little girl? At bath time she totally lost it, once again said she hated me and what was the point in being part of a family if she wasn't wanted. She said she was going to run away and she didn't need any extra clothes because she wanted to die. Then we'd miss her. Taylor argued with her saying she thought of running away before, but think about what she does have; a warm house, lots of toys... I'll skip some details. In the end Kenzie said she just wanted to be told I loved her and that I'd make certain Taylor didn't ruin her birthday when the time came. I told her I love her, but I hate her behaviour lately and that I couldn't promise to keep Taylor from ruining her birthday because it wasn't a promise I knew I could keep. She disappeared into her room for 5-10 minutes and came back out her old self again. She's only 7 years old. What am I in for when her brain is going through chemical changes and is 17 years old? Breathe in, breathe out. One day at a time.