I would vote for the first, shorter possible thesis that you present: “The art of losing isn't hard to master, it's up to you if it's a disaster.” Your follow-up discussion inside your question shows that you’re thinking a lot about the speaker’s reflection on the significance of each loss...
I would vote for the first, shorter possible thesis that you present: “The art of losing isn't hard to master, it's up to you if it's a disaster.” Your follow-up discussion inside your question shows that you’re thinking a lot about the speaker’s reflection on the significance of each loss and about the speaker’s/writer’s new awareness at the poem’s end, so I think that a thesis stating that “it’s up to you” is right on target. That person in the poem who loses things is the one who assigns the significance of each loss, and that person may not always be honest with herself or with us until the losses have really piled up.
You explain how you are going to walk through the poem, tracing the movement from the loss of small things to the loss of things of more importance. In tracing this movement in the poem, you may also be able to trace both a shift in tone in the poem and a shift in the form of the poem. For me, the tone seems to start out as playful or flippant, perhaps even mischievous or gleeful. For example, in the second stanza the speaker suggests that the door keys wanted to be lost, it was their “intent” and, thus, losing them was really not the speaker’s fault.
What happens to this playfulness through the middle section of the poem? How do we, as readers, make sense of the speaker’s suggestion that we should even “practice losing farther, losing faster”? How does the exclamation “Oh look!” sound when we read it out loud?
Even the form of the poem seems to present the subject of loss in a light way, with easy, repeated end rhymes up until the final stanza. (Using “last, or” to create an end rhyme with “master” strikes me as really playful, too.) The final stanza has more lines and more insistent rhyming (ABA becomes ABAA). What does this shift from three lines to four lines signal? How does the final stanza (in both content and form) stand apart from the rest of the poem?
You’ve already done a lot of thinking about this poem. Your attention to the meaning of that quick parenthetical comment in the last line -- (Write it!) -- is great! Good luck with your essay!