Your body is expending a TREMENDOUS amount of energy to heal the sternum. This will be quite evident until 6 weeks out when you should experience more energy for exercising (but you will still tire fairly easily). As Nancy noted, you will continue to heal (more gradually) for 6 to 12 months before you feel you have fully recovered.
I like the plan presented by Bob Lowe, walking for 5 to 10 minutes, three times a day, with NAPS in between and gradually step it up. That is a more realistic approach IMHO. As everyone has said, there are Good Days, and Not so Good Days. As long as you can see progress on a weekly basis, you are headed in the right direction. If you have a continual decline, you need to call that to the attention of your Doctors (Surgeon, Cardiologist, and PCP).
If your insurance will cover Cardiac Rehabilitation, I highly recommend it, starting at 6 weeks or so post op.
'AL Capshaw'