by the time Jin finds her, Fuu has already made up her mind to leave them and go on alone, which she does in the next episode. That's why she seems so wistful, that's why she tells Jin she's sorry, that's why she doesn't want to discuss any future plans or concerns of Jin's: she believes that by this time tomorrow they'll be separated for good. She knows it will hurt them, and she's sorry, but she's made her decision. I think this is the simplest explanation that fits all the facts and doesn't go out of character for anyone, and as far as I'm concerned, I'm sticking with it.