Is it considered backbiting if you tell your friend you want a divorce from your husband/wife? But you already told your husband/wife that you wanted one.
This is best answered by what quoting the Islamic definition of backbiting, you can use this as a criteria for assessing whether something is considered as back biting or not:
It was narrated from Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: “Do you know what gheebah (backbiting) is?” They said, “Allah and His Messenger know best.” He said, “Saying something about your brother that he dislikes.” It was said, “What if what I say about my brother is true?” He said, “If what you say is true then you have backbitten about him, and if it is not true, then you have slandered him.”
The Messenger of Allah (sallallahu 'alaihi wa sallam) said, "Do you know what is ghibah (backbiting)?" The Companions said: "Allah and His Messenger know better." Thereupon he said, "Ghibah is talking about your (Muslim) brother in a manner which he dislikes." It was said to him: "What if my (Muslim) brother is as I say." He said, "If he is actually as you say, then that is ghibah; but if that is not in him, that is slandering."
Those two questions contain more details on backbiting: