One of the most difficult and therefore most painful aspects of exercising is doing the activity even though you are tired. Many of us schedule our training for the end of the day. Usually after we come home from our work day, (yeah, most of us have to work for a living), we pull from deep within ourselves and do the deed. For me it is often a painful experience albeit psychologically.
But you need not suffer. From that place where you found the inspiration to push through the 'pain' is a storehouse of positive feelings and emotions. We have a certain capacity to manage our perceptions and to focus on the good, so much so, that for many it is the only effective mechanism for keeping our healthy living plan moving forward.
Dig deep - dig often. Don't cave to some temporary fatigue. You know that when your done you'll feel great and be thankful that you persevered. (Of course, if you feeling the effects of over training then it is prudent and wise to take a rest day.)