Is it considered forbidden for a person to talk to themselves internally in a way that displeases another person, or to talk internally about an oppressor without moving the tongue, and how can one vent about problems without falling into backbiting?

Intrusive thoughts are forgiven as long as one does not speak them, act upon them, or firmly intend to carry them out. It is permissible for an oppressed person to speak about their oppressor to someone who can remove the injustice, or by way of retribution. Allah Almighty says: "Allah does not like the public mention of evil except by one who has been wronged." (An-Nisa: 148) Similarly, there is no harm in a person discussing their problems with others, as long as it does not involve forbidden matters.