In just a few lines, the speaker describes themselves as a crowd of things that have seen better days. "A society of worn instruments" suggests many parts of the self, all tired from use, like tools that once did good work and are now dulled. It's a harsh way to look at yourself, but the word "society" also hints that there's a whole community in there, not just one failure.
The last lines get more painful. "A should-have-been" sums up regret: a person who didn't become what they might have. Then the image of a "rogue fleet gone into ink" pictures ships that slipped their orders and vanished into writing. The speaker may be saying that their energy and ambition ended up as words on a page instead of real voyages or deeds. The mood is wistful and self-mocking, with a hint of grim humor. The title, "Self-criticism," fits, though the sea imagery gives the disappointment a strange, faded grandeur.