Sometimes we are our own worst enemy. At least in my case I know I am. I think I should be able to do that - nope I can't and then I'm upset. That is all part of a TKR I believe, its the part you don't hear about until your going through it yourself! Slowly you learn your limits, your knees limits and then healing and life get somewhat easier.
Resting during the day is just that, resting. Or napping, if you need it. That's your body healing and telling you to stop and let it do its thing. There is nothing wrong with doing both several times a day. I always felt guilty when resting so I made sure to 'do something' when I was up. Mind you it was always something small - a new kleenex box, get a can of dog food out for later, start a grocery list, etc. Just something and then I didn't feel guilty because I needed to rest. I realize its all a mind game, but that worked for me.