--Even losing you (the joking voice, a gesture I love) I shan't have lied. It's evident the art of losing's not too hard to master though it may look like (Write it!) like disaster.
This is the kicker. Here, we discover that the loss the speaker has been mulling over this whole time is that of a beloved person.
We’re not sure exactly who this person is, or what his or her relationship to the poet is, but it’s clear to see that this is one loss that the poet hasn’t mastered.
She allows herself to remember some of the things she loved about the addressee – a voice, a gesture – and admits, finally, that the art of losing actually is hard, but not too hard, to master, and that losses do sometimes look like disaster.
Her mini-breakdown in the last line (the repetition of like, and the interjection "Write it!" [6.19]) demonstrate the true difficulty of coming to terms with loss. For the first time in the poem, we see her façade of confidence and good humor disintegrate; the fact that she has to force herself to even write the word "disaster" this last time reveals the poet to be deeply human and vulnerable, just like the rest of us.