Stay with the feeling as you would a friend; if you are sad, you feel sad but you can give it space by realizing it is not "you" it is just your feeling. Like sunrise says, you don't try to get rid of it in some way or change it into something else, you "listen" to it and see what it might be saying, what thoughts it might bring up ("I remember feeling this way yesterday when John seemed to be ignoring me") and what other feelings there might be associated with it ("sad" is a rather vague word that can mean anything from out-of-sorts to grief stricken Sad Synonyms, Sad Antonyms | Thesaurus.com). What might be missing? If John seems to be ignoring you, were you also disappointed or angry about that?