Give yourself a timeout.
Step away from your desk, have the hubby take the kids, do whatever you need to do to give yourself some time alone to decompress.
Smiling, even when you don't feel like it, is proven to boost your mood. So even on your crummiest days, keep smiling and eventually you will actually start to feel better.
As if you need a reason, but eating chocolate causes the brain to release endorphins, endorphins make you feel good, etc., etc.