Marian, you've had a rough time of it. Your stamina will return, and I love that you've given your PT your specific goals to work toward. Having the confidence to get back up after a fall is crucial, and it's an excellent goal to work toward. Make sure before you "graduate" that you're able to get down on the floor and back up while at PT so you'll have the confidence to do it when you're alone. I know you'll be monitoring your stamina, because that seems like it's going to be your critical factor in your successful return to work.
Consider that your body has been using tremendous reserves to get you healed not only from your TKR but also from the infection. Both of those require a lot of work from your body. As your healing continues, I think you'll find your body is really in pretty good shape -- it's been running a lot of marathons on your behalf. Soon you'll be able to shift your attention to things external from healing, and that stamina will be there for you. It's likely to take a few more weeks, though.