I don’t find it obvious at all that Yoda is speaking of his failure in a grand, centuries-long sense. Sure, he could be, but it’s equally possible that he’s referring merely to his failure to kill Palpatine, which was the interpretation I had watching the film.

Ultimately, interpretations are formed based on the filmmaking, and there’s little to suggest Yoda’s thought process here. Minutes before this he was self-assuredly walking into the Senate saying how Palpatine’s rule was about to end and his faith in Vader and the Dark Side was misplaced. These words may be bluster, but at the same time they don’t reveal a deep conflict within Yoda. He seems to only question himself once he loses the fight, at which point he has decided to run off into exile before the speeder has even left the air parking lot. Maybe if we saw him gazing out at the still-burning Jedi Temple, the virtual camera lingering on the sadness in his eyes as he comes to a conclusion, it would be clearer that he views his failure as something more than losing a single fight. But that’s not in the film, and I don’t see why I should do Lucas’s job for him.