The interesting thing I found in her speaking of losing items is in her choice of the lost items she speaks of. Bishop talks about both losing both tangible items, as well as intangible items. As the poem continues along, her lost items become more intangible. She first mentions lost door keys, her mother's watch, and forgetting names, which I would consider a pretty definite thing like an object. She then gets into more figurative items. She speak of losing "cities, lovely ones," and moves on to rivers, and a continent. She follows speaking of these large vast things in saying "i miss them, but it wasn't a disaster." I believe she is saying that even losing very large things such as this that no one person would be able to own aren't worth stressing about when losing.
The last stanza finishes the poem with a sarcastic tone. She opens it with "Even losing you (the joking voice, a gesture I love)." She jokes around, saying even if you lose yourself it isn't a huge deal. The last couple of lines are "It's evident the art of losing's not too heard to master though it may look like (Write it!) like disaster." The new imperfect sentence structure could indicate that she herself is stressed and/or lost part of sanity. This woman may have finally become so fed up with losing her possessions that she is attempting to convince herself that it is no big deal to lose things, and that they aren't important.