You're back at work already, Dragonfly? Oh, that is really early. It's much too soon for your poor knee.
Your surgeon is an incurable optimist, who has obviously not had a knee replacement himself! He's set you up with unrealistic expectations of your recovery time which, I am sure, is partly the cause of your frustration right now.
Even for a partial, we recommend taking about 12 weeks off and then going back part-time initially.
Phased return to work
Your tiredness is caused by the energy drain, which will be with you for some time. You see, most of your body's energy is being directed towards healing your knee and there isn't much energy left for anything else.
Energy drain for TKRs
However, you're back at work and it is what it is. So, on days that you work, try not to do anything else at all. No cooking, no shopping, no PT, no exercises - work, plus getting to and from your work place, is more than enough exercise. Just come home, grab a quick snack and then spend the rest of the day resting, elevating and icing your knee.