Why don’t you look at repertoire that is better suited for your hands? Gary Schocker has written some fun yet musically appealing works like Lessons I Learned from My Cat, My Kingdom for a Harp, Quest-Reunion, or Waves that will give you a chance to make a splash, but not overly tax your hands. Also, Tournier wrote some smaller works that are very gratifying to play like Berceuse Russe, as well as Grandjany and Salzedo. Look at Hasselmans’ Guitare, Patrouille, or Reverie. Beautiful repertoire.
Build your strength. The piece is something of an endurance test. You cannot put cuts in it. You can practice it as block chords a good part of the time. Ibert’s Reflet dans l’Eau is even longer. Hasselman’s Prelude is wonderful and gives you the reverse pattern, of upward arpeggios, but with very wide spacing. So, figure out how much you can practice without fatigue, and do that much each day, changing sections, and when you have it up to tempo, you’ll be able to put it all together. It’s not logical to try to play through the whole piece in a slow tempo. Exercising your arms with weights is also very helpful in building strength, stamina and endurance. It is a professional piece, though used widely by students.
Julie, the easiest way to shorten La Source is to begin at the recapitulation and just play to the end of the piece. This avoids the D minor section entirely, and you could even start with the Introduction before playing the recapitulation, to make it a little longer. Hope this helps.