Complex sentence: Her self-portrait shows much about the subject, but certainly not the floating beauty that Rossetti painted and is historically significant because it shows, through her own eyes, a beauty who was idealized by so many famous artists.
Split into simpler sentences: Her self-portrait shows much about the subject, but certainly not the floating beauty that Rossetti painted. This painting is historically very significant because it shows, through her own eyes, a beauty who was idealized by so many famous artists.