Hello there,
How are you getting on, a year later? YOu did not give your age: are you likely to be still growing or are you a matured adult?
I don't believe your problem is lack of finger length. I think it may be that your muscles and tendons are [or were, a year ago,] not yet used to the 'feel' of the chords.
TO get your hands used to feel of the basic chords, you should be practicing arpeggios up and down the keyboard. I have a formula which I use which enables me to play every 1-3-5 arpeggio in every key, major and minor, in root position and 1st and second inversions, without getting too boring. After a lifetime of ignoring exercises last year I went to a new teacher who immediately got me going on scales and arpeggios. Dramatic improvements followed quickly!
You know that this piece was originally published in G major, with Schubert's agreement, even though he wrote it in G-flat major? I was given it to learn in G-major when a child; and it was years before I discovered that it was "supposed" to be in the 6-flats key! You may find G-major chords a little easier to cover?
But if you hands are really physically too short for this particular piece then you should move on to something different. In the meantime learn, say, Mozart's sonata in Bb K333. There is hardly a single octave in it. Or the slow movement of Beethoven Pathetique, if you haven't already found it, which is not dissimilar to the schubert, but with smaller-compass chords.