I'm up to my neck in work.

Let's take a coffee break, shall we?

I wish I could, but

What keeps you so busy? You've been sitting there for hours. You've got to walk around. You can't just stay on computer forever.

Well, I'm up to my neck in work. I've got to finish this report. Sarah needs it by noon. I don't want to be scolded if I can't finish my work by the deadline.

I understand that, but you'd feel better if you took a break, even for just a short while.