Do you ever feel completely drained at the end of the day but you just can't put your finger on a specific cause? It's quite possible that there is more than one cause or it could simply be that you're facing one particular stressor that is slowly draining your energy.
Here's a task. During the course of your day, take time to really consider how you're feeling. Take a break every 30 or 60 minutes and gauge your energy level. If you're feeling drained, make a note of what task you were involved in and when you have more time, stop to figure out why it drained you -- did it take a lot of mental energy? Did you say yes to a project for which you should have said no? Are you involved in a task that brings you no mental or spiritual joy? Have you been assigned a project for which you simply don't have the skill set necessary to complete?
Gauge your mood and your feelings for a couple of days to get a good read on your energy-sappers. If possible get those off your plate. If it's not possible, go into the task knowing it will drain you and give yoruself a treat once you've finished. By treat I don't necessarily mean a piece of triple layer chocolate fudge cake, although that may not be bad for an occasional treat!
Have you uncovered your energy-depleters?