A short poem like this one does a lot with very little. The speaker says that "hope" is a four-letter word, which plays on the way we usually use that phrase for swear words. Here, though, the word that gets the rough treatment is a gentle one. Hope is what kept the speaker going after love "let me down."
The feeling underneath is a mix of hurt and quiet stubbornness. Love, the thing that was supposed to hold the speaker up, failed them. Hope is what remained. There's a little bitterness in the joke, since the speaker seems to be saying that the word everyone treats as soft or naive was the one with real strength. Notice too that "love" sits alone on its own line, almost like the drop of being let down.
The mood is wry and tender at once. It isn't a grand declaration of recovery. It's a small, honest admission that when something big collapses, a modest word can carry you through.