Isn't it hard when you know your child can do so much better, isn't it? It's hard to give advise since each child is so different. But, I can tell you about my son.
I my son's case, he is having a hard time with math because of lack of practice. In fact, my husband had a really long and loud talk with my son about math last night. My son seems to think the information will just jump into his head and he will make As on his math tests. My son is in 8th grade, but taking 9th grade algebra and, yes, it's hard. We figured practice is the only thing he is missing. He is making all As on homework, but just can't seem to get it together on his tests. But, when he spends some time working through problems over and over again, he does much better.
So, that's my story. Maybe your daughter just needs more practice. Maybe extra practice problems from teacher? It could also be just a phase she is going through. In fact my son's English teacher told me he is acting more like a teenager this semester, which might be the reason for his lack of good study habits. Not a good thing to tell a mother. LOL.
It maybe too late to go back and make a new start, but it's not too late to make a new ending.
I eat to live. I don't live to eat.