If one were to speak loshon hara to themsleves about someone, is it allowed? Of course it is not ideal, but if someone needed to talk to blow off steam, would it be something that is permissible?
In contrasting inappropriate speech with lashon hara, R. Judah Loewe writes as follows:
רק החטא הוא מפני הדבור עצמו ואפשר שאפי' אם היה מוציא דבר נבלה בינו ובין עצמו היה חטא דלא דמי ללשון הרע שהוא מדבר רע לאחר ואין עיקר החטא מצד הדבור רק מצד הרע שהוא עושה בדבור
The sin is just because of the speech itself, and is is possible that even if he spoke inappropriately to himself he would be sinning, for it is not comparable to lashon hara where he speaks evil to someone else and the essence of the sin is not the speech but the evil that he commits with the speech.
He seems to clearly assume that lashon hara is not applicable when you talk to yourself, since you are not perpetrating any evil. This is as opposed to inappropriate speech which is inherently bad, and thus problematic even if you are only talking to yourself.
Rabbi Gil Student writes in his sefer "search Engine" page 273-276 about the idea of writing letters to vent about others . He brings many sources and concludes that one may write letters for their eyes only to calm oneself and to do in a constructive matter ,but if done just in anger seems more problematic for the reason of bearing a grudge. He also cites that saying lashon hara seems to apply only when said to another person not when alone .
See this answer which quotes the Chafetz Chaim (6:4) who allows one to listen to someone else vent their frustrations as long as the listener does not accept the words as true. (In fact, the Chofetz Chaim goes as far as to call this a mitzva!)
Aside from the permission to vent to oneself about someone as long as you don't accept it as true - by judging them with the benefit of the doubt - there is an additional leniency for this individual case. If there is truly no one else who will be hearing you vent, the Chofetz Chaim (2:1) says
אָסוּר לְסַפֵּר לָשׁוֹן הָרָע עַל חֲבֵרוֹ, אַף שֶׁהוּא אֱמֶת, אֲפִלּוּ בִּפְנִי יָחִיד - It is forbidden to speak lashon hara against one's friend, even if it is true, even before one
The greatest chiddush is that it's even forbidden to speak to just one other person, but if you would be speaking to yourself then it would seem to be permitted. Nonetheless, that does not preclude our obligation to judge others favorably.