The line Joe Biden used to put into nearly every big speech — “I’ve never been more optimistic about America’s future” — is a long way from what he says in private now. These days, multiple people who’ve spoken to him over the last year say, Biden often punctuates conversations with: “You think we can actually come back from this?” The 83-year-old Biden continues to feel out a post-presidency that may prove to be one of the shortest in history and is already one of the most complicated. There are days when Biden is heartbroken, indignant or in disbelief about...