How Do You Deal With Teens Attitude?

So, I am struggling a bit today with a certain teen in our family that is truly a great kid, however her attitude bothers me and I am not sure how to deal with it? Do I ignore it, try to reason with it, argue with it?

My grand that I raised for most of her life with the exception of the past year and half or so, turns 16 soon and if you don't think that is a milestone, let me tell you, she fluctuates from not wanting anything for a gift or party but cash to wanting to know if we are doing something for that day?

I had set it up for her to help with a friend over this past Sunday and she cancelled on her after planning this out for the past few weeks at the last minute. I tried to reason with her, but she shut me down and out quickly. Was I to ruin a surprise and tell her she was working with her to find out what type of cake to make her?  No! I just let it go.

Her dad is now working out of town this week and has left his girlfriend in charge of there place and the kids. However, she has no paperwork at all should they get hurt, she doesn't drive so she can't pick them up from school functions, etc.., it still falls on my shoulders.

This morning, I advised her a young man who has hurt her in the past was not who she would be hanging around with. Her reply?
'You can't tell me what to do, you aren't my parent.' I told her I understood that, however, I was the family adult and driving her to school and back and I did have some say. Her reply? 'If my dad wanted you to be in charge, he wouldn't have left his girlfriend to watch us.' 

Then she accused me of not getting her home in time to shower last evening (she was home by 830) and that taking a shower in the morning isn't an option because she sleeps in and I can just stop talking to her.

I'm hurt and more than disappointed. 

She has recently taken drivers education and now is telling me she will probably fail the test because I don't let her drive enough. I laughed! My car got hit a few weeks back and was in the shop and she could not drive the rental.

She puts no effort to drive when I am available, wanting to hang out with friends instead, so I am confused.

I guess the real question is, how do I just let go?
I no longer raise her and her siblings. They no longer live with me. How do I just let go and learn to tell them no?