Is informing close friends about problems or annoying behaviors to relieve psychological pressure considered backbiting (gheebah) or slander (nameemah), even though what is said has already happened?

Backbiting is mentioning someone in a way they would dislike in their absence. It is forbidden except in certain exempted cases, such as mentioning the injustice of an oppressor without exceeding bounds. If you have been wronged and do not overstep the limit in your speech about them, there is no sin upon you. As for tale-bearing (namimah), it is conveying speech between people with the intent of causing mischief, and this is not applicable here.