The home key of the song is F minor - which is confirmed at the beginning and at the end of the song; but in fact we are 'drifting' all the time between this home key and its parallel major key Ab major. This 'drift' is undoubtedly related to the text: As so often with Schubert contrasting major and minor keys symbolize the contrast between a (happier) past and an (unfortunate) present. See for example the contrast between Ach wer bringt and die schönen Tage ...der ersten Liebe  and ach wer bringt and nur eine Stunde jener holden Zeit zurück. And in the passage from Einsam nähr ich exclusively the present is described - so, we stay in F minor. At the absolute low at the end of this section (in the vocal part literally as well: the lowest note of the song) at ums verlorne Glück, the dominant of F minor is reached via an altered chord, so to underscore the drama. Then, the beginning of the song is repeated (abridged), and thus we find a cadence in Ab once more (in the penultimate measure). But in the last measure the piano causes a return to the sad present, by reintroducing F minor: