dido begins by singing a recitative, which i suppose is the main motive in which the later voices vary. this main motive is quite slow moving and depressing (what a giveaway the title was...). the melodic interest lies in the first 4 measures, where she sings "thy hand, belinda! darkness shades me, oh thy bosom let me rest."
later, in measure 14, (where dido re-enters), she produces a slight variation on this theme, especially noticeable in measure 16. in this measure, she has a descending line, quite like the one seen and heard in measure 2. also, measures 17 and 18 are quite similar to measures 4 and 5.
when dido finishes singing her first variation, she moves on to a second, where she continues to use the familiar dotted rhythm, but a rather different melody. the accompaniment, however, replays the motive that dido just finished. the harmonic movement remains relatively untouched, which i feel provides some stability for the listener.