How to handle working mom guilt

How can I stop feeling guilty and resist the temptation to be "Superwoman"? I'm a working mother with a preschooler and a couple of school-age kids who are also involved in lots of extra-curricular activities. I have to admit that I feel bad whenever I'm not doing something "productive." My family's schedule is so tight, and there's so much to be done, that I feel compelled to "work" – and sometimes that involves multi-tasking – all the time. If I don't, I quickly become gripped by the fear that things won't get done.