The speaker has just realized they have no best friend, no one they can truly open up to. They had believed they did, imagining friends who would accept them without judgment and understand everything about their life. That belief turns out to be mistaken, and the poem treats the discovery as a sudden, painful shock.
What hurts most is that the same people who let the speaker down also shook their faith in the idea of "best friends forever." The quotation marks around the phrase suggest the speaker now sees it as a naive promise rather than something real. From there the loss widens. The speaker feels they are slowly fading, "bit by bit, part by part," until they end up as dust that nobody, not even they themselves, can recognize.
The mood is lonely, quiet, and resigned. The trailing dots at the end make it feel unfinished, as if the speaker's voice is itself disappearing.