they work so hard at school... and being "on" and behaving.
When at home, they need to wind-down.
maybe let her relax more first before starting in on homework. Start by giving her a head's up: "After you finish your snack, well start homework..."
"transitioning" the child into the next thing you need her to do.
Some kids, don't do well with abrupt changing of pace... at home, is where they get tweaked after a long day at school. Literally.
My girl, does NOT like to start homework, until she has eaten... not while eating. And I always give her a head's-up on when homework will commence. For instance: "when the clock says 3:00 its homework time." Until then, I let her snack/eat, relax as she wants. It is winding-down from school. Deflating if you will. I know she needs to do that.
If it is too hard as she says, ask her "why." And she probably gets all frustrated because she is tired. Most kids are, after school, and then they can't concentrate.
But, if your daughter is really having work that is too hard... and she is struggling, then you need to talk to the Teacher. See how she is doing in class.
Also, try assisting her differently. Sometimes, my daughter just gets more irked the more I assist her. If she has a question, then she asks me. But I am there, sitting beside her, "helping." So I've learned her cues, and her pacing. And not rushing her.
My daughter, would literally be just all pooped out when she came home from school, and would still nap through the 1st grade. Until she was refreshed, trying to get homework done, on my part, was just a struggle.
All the best,
Susan