Try to keep your cadence consistent. Spin a little faster than you feel is "normal", and you will soon see that the rhythm and ease of pedaling comes naturally. That will likely help with a lot of your exhaustion issues and maybe with your peaks and troughs of energy.
If your sit bones hurt, is your saddle too high? Is your down pedal leg completely straight when you are sitting? Do you rock in the saddle? If so, try lowering the saddle by about an eighth of an inch and see if the comfort level improves. Conversely, if the saddle is too low, you can get sit bone or tail bone issues.