it's the last line.
" She had the oddest sense of being herself invisible; unseen; unknown; there being no more marrying, no more having of children now, but only this astonishing and rather solemn progress with the rest of them, up Bond Street, this being Mrs. Dalloway; not even Clarissa any more"
Explanation: Basically, after all that was said and done, it was just her. No one else. Nothing else to prove.
By the way, this was correct in Plato.
the section in the paragraph that illustrates Mrs. Dalloway´s feelings of emptness caused by her social identity is:
She would have been, in the first place, dark like Lady Bexborough, with a skin of crumpled leather and beautiful eyes. She would have been, like Lady Bexborough, slow and stately; rather large; interested in politics like a man; with a country house; very dignified, very sincere