Paradoxicaly, no matter how I feel, once I step onto the mat, I seem to rally the energy to complete the task ahead. That's the funny thing about 'energy' - when you put out the effort, and the sweat starts flowing, you always feel better for doing so.
It seems that there is a universal law at play here - the more you give, the more you receive - the more you put in, the more you get out. Training in the martial arts has always been an uplifting and highly positive experience for me - one that always leaves more fully charged for having participated. Every day I train - is simply a better day.
When in doubt - simply step onto the mat.