Your post has several contradictions so it's difficult for me to make any suggestions. Read the following and help me understand the difference between;
"Thing is, she has this thing about her where she is never satisfied with what she is given. She constantly asks for things. ... She just goes from one thing to another, ... she’ll ask can we make ... Then she’ll come back and ask ... After the visit she’ll ask ... On the way home she’ll ask ... This girl just never stops. It goes on and on like this... If you say no, she’ll keep asking even if you tell her not to ask again. If you say I’ll think about it, she’ll hover over you until she gets an answer. I’m so glad that school is starting, but I’ll still have to deal with it after school and on the weekends."
After reading this far I thought I had an accurate picture of what you're dealing with but then you said;
"Just in case you’re thinking it, she is not privileged nor has she been spoiled. She does not get her way all that often either."
JQNA, I can't get there from here. She's either spoiled and totally determined to get her way at all costs or she's not. If your answer is yes, Oron has some excellent suggestions, but you need to commit. Either she needs professional help or you're adjusting to having a teenager around the house and there is a difference.
Is it possible that some of this is you?