You are describing my feelings to a T...when I'm suffering a major bout of depression. Stupid MS causes this too...
Have you looked into trying an antidepressant? I go on and off Cymbalta (with doc's supervision) when I'm having trouble digging out of the hole I feel I'm in occasionally.
Also, in the winter when the sun's hiding, I use a full spectrum light, because I know for a fact I suffer from seasonal affective disorder.

And as much as I do NOT feel like it, I make myself work out, whether it's taking the dog for a walk, working out with Wii, or going to the gym. It gets the endorphins in the brain pumping, making me feel better.

Make sure to address this with your doctor-he/she needs to know that this is going on!! It is a physical reaction, not just "I'm tired/in a bad mood etc".

Take care of yourself...