Complex sentence: Sackville-West called the marriage her own greatest failure as she made a plan for Keppel to escape with her to avoid marrying Trefusis, but was too cowardly to carry it out.
Split into simpler sentences: Sackville-West called the marriage her own greatest failure. She had made a plan for Keppel to escape with her to avoid marrying Trefusis, but was too cowardly to carry it out.