My heart goes out to you as you struggle so much with this recovery, especially after having a very successful recovery with your other leg.
I think you still get so tired so quickly because your body is still trying to heal, and has been for a long time, with these last surgeries, plus the infection. With all the swelling you have, it’s going to be a while yet, before you can build yourself up. I believe that will happen when your healing is able to make more progress. Try to hang in there and believe that you will indeed progress, in time.I get tired so quickly so I need to build myself up.