Can a person speak lashon hara about someone to a spouse or parent in order to relieve oneself of emotional tension? The parent or spouse will not retell the lashon hara to someone else. For instance, an obnoxious boss or person is frustrating you. Talking about it can help, altough the talking is derogatory.
[This is a delicate question, and I thought at first to offer a lengthy answer, with many sources. Unfortunately, I have not had the free time to do this, and I therefore settle for a short answer for the present. Please G-d, I will write at length on the subject in the near future.]
Lashon hara is an exceptional Torah prohbition, in that it is permitted to speak lashon hara when doing so is leto’eles, for a constructive purpose (see Chafetz Chaim, Kelal 10, quoting from Rabbeinu Yonah in Shaarei Teshuvah and in his chiddushim to Bava Basra). The question is asked: how can a prohibition be permitted for the purpose of some constructive benefit?
The answer to this question is that when spoken for a constructive purpose, the speech, although it may include derogatory information about somebody else, is not classified as lashon hara. The Vilna Gaon writes (commentary to Esther 10:3) that lashon hara is a bad character trait (middah ra’ah), just like anger, pride, and so on. In this case, the negative character trait is hate and spite, and its manifestation in the form of ‘evil speech’ is considered by a number of authorities (including Rambam) to be a Torah transgression.
However, when spoken out of a positive, constructive purpose, and not out of hate and spite, the speech, even with its derogatory content, is not lashon hara. This is why it is permitted to speak lashon hara for a positive purpose, such as for people to keep their distance from damage (or from a wicked person), or to chastise the wicked (the two are mentioned by Rabbeinu Yonah).
In the same sense, it is also permitted to speak lashon hara when this is required for relieving tension. A highly negative experience can cause much harm, and sharing it with a spouse can be essential to getting over it smoothly. If the negative tale is required–a shower/sleep simply won’t do the job–and it is related in the spirit of relieving tension, and not as hateful and spiteful speech, it would not be included in the prohibition of lashon hara.
Note, however, that this applies only to the negative/derogatory speech element of lashon hara. There is another element of the prohibition, in that it is forbidden to cause a person damage (e.g. financial damage) by means of speech (see Tosafos, Zevachim 88b). Even when speaking the matter over with a spouse is required, one must therefore be careful that the speech will not be actually damaging to the relevant party.