The larger form of this piece is Ternary, your typical A B A, and this form is made doubly obvious by the seemingly strange key change from Ab major to E major. Actually, this is closer than it first appears. Schumann must have read about Beethoven and his crazy chromatic mediant fetish; E major is an enharmonic respelling of Fb major, the flat major VI to Ab's tonic I.
The key change itself is a text-painting device: as we approach the key change (remember, down a third from tonic) the singer's line is "you my grave, [down] into which I cast my grief." And then we change key, using a nifty common-tone modulation, Ab-->G#, and the accompaniment becomes less florid and more supportive, grounded. The singer explains that "you are rest, you are peace, you are bestowed upon me [down] from heaven." The accompaniment is more solid and chordal - if it were minor, I would say it's solemn like a funeral march - the major gives this much more hope and joy, even though we're kind of still in the death-theme textwise.
Then, just as cliche and text-painting-y, the key changes back (up) to Ab (with another common-tone modulation, C#-->Db) on the perfect line, "you raise me lovingly [up] above myself." Oh Schumann. How cute is that. (PS - anyone else hear the Josh Groban song with that line?)
A final interesting note is that one Schu here steals from another Schu - part of Schubert's "Ave Maria" is quoted in the last measures, twice. The do-ti-do-mi-re-do of that melody is set in a ground-bass type of ti-la-le-sol-do falling progression. The chromaticism describes an otherworldly serenity and happiness, as if the singer is still in disbelief at their luck and contentment.
And that's all for this piece. I will almost miss blogging with you guys. Have a great summer, all! Ciao!