It was pretty vague at first but the song is originally written from a man's point of view to a woman (1880 - Juan de Dios Peza)
http://www.poesi.as/jdp310.htm
The song in the video is only a handful of stanzas from the original, and the women seem to switch that point of view in the video. But if we go with "his last letter", he's telling her he's ready to stop looking for her in mirrors and look for the real her wherever she may be.
*= chose masculine term but feminine could fit just as well.