Sometimes it is a good idea just to schedule your down time. In other words, if you are feeling down and depressed, decide for how long you want to be in this state. Tell yourself: 'For the next three days I am just going to be out of it. Then I'll go for it again.' Then take time off. Be depressed and demotivated. Don't feel guilty about it. You have scheduled it. "Enjoy it" Knowing that when the three days are up you will feel better, see things differently and be ready to face the world again and be productive.
This approach has REALLY worked for me. Make it work for you too.
Its OK to be down sometimes
I'll feel better soon enough
No one will die, while I take a break
Be all you can be.