Who are you and what have you done with my daughter?

The mouth on my eight year old daughter is unbelievable!!! She is in a mode right now where she argues EVERYTHING with us, talks back, doesn't do what she's told, and is a overall snot!! I knew these things would happen when she was a teenager, but at eight really?!?! I hate to confess this but I times I can't even stand to be around her, it's that bad!! I told her today that she had to journal every night (her teacher's request) she flipped a lid!! Olivia spent a good hour crying, yelling and arguing with me about it!! She insists that she is not going to do it, it's stupid and she says she'll "NEVER do it"! Ugh..I would love to explain to this lovely child that I mentally cannot handle having one more of these "arguments" with her. I boldly tell her that she is going to do it, that this is not a choice. No matter what I say or do she flies off the handle and goes nuts!! I know something needs to be done, I'm just not sure what. We ground her and send her to her room, but none of these things affect her at all!! I need help...any advise??