A Mental Change

I quickly learned when I got pregnant that my "normal" work mentality needed to change.  I was no longer going to be able to stay at school until 6 o'clock at night, and spend countless hours on the weekend prepping and searching for materials for my students.  I was now going to need to focus on a new life.
I am happy that I realized this early on in the pregnancy, but now that I am back to work full time I am having a hard time not getting upset with myself for my never ending To Do list at school.  As I rush out the door to pick up my son each afternoon, I get this quick rush of anxiety as I try and figure out how I am going to get it all done.  Thankfully, the moment I see my son it all disappears and my focus is on him.

How can I be sad when I see this face?
I need to know, how do all of you working mamas do it?  How do you keep up at work and still be present at home?