Sharing the position of the narrator, I'll try my hand at interpreting as well.
"So here goes" starting off is very much a phrase used to indicate reluctance in completing some action, a lack of confidence, motivation or desire to do the thing. In this case, I believe it's reluctance for lack of confidence and motivation--the narrator has the desire but "one last letter now, one last attempt to make sense" indicates he's losing hope in his ability to write to clarify his existential angst for himself or for others.
"What have I been trying to accomplish?" is not a literal misunderstanding of his action, but the emotional and mental fatigue from the intense existential thought clouding his mind as he's wandered through a labyrinth of uncertainty and profound doubt. The attempt to uncover the meaning and represent it makes things "blurry"--very comparable to the painting in Virginia Woolf's "To the Lighthouse", where the character Lily Briscoe attempts to find and represent truth and totality through art, a meaning, a reason, an essence, but paints a blurred picture. Life's many intricacies and tragedies are incomprehensible in finding a comforting reason, or a reason at all.
The narrator is having an identity crisis. He's adopted a skepticist mindset where he questions his very own self, because he's lost his own normal grounding in his self-defeating, self-examining behaviour. He recognises this shift and no longer knows if he is the same "voice" writing these stories, not as if he's changed as a person but rather that he is a stranger to his former self, his normal life, before the "departure". He doesn't even know if he's always been that way, that he's just uncovered his real self. It's schizophrenic and character, yet I honestly can't tell if it's metaphoric or it's simply a result of his existential depression.
"I don't know... I don't know." This is a melancholic, exhausted admission of defeat, not a cathartic acknowledgment of uncertainty....
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