Fun little story! I liked that it jumped right into things and had a clear objective and conflict (a lot of the things posted here don't). The use of personification and physicalizing metaphors around writing worked perfectly to give it a parable-type feeling and sense.
One thing I was a bit unclear on was the resolution. I think I get that the key to finishing the story was realizing that the author's journey was in some way the character's journey, but I wasn't sure I was reading that right or what the revelation in the mirror really meant.
Let me be clear: the last thing you need is a whole bunch of explanation. Leaving it vague is fine; the general idea is clear enough (he struggles, he has some insight that rekindles his drive, he wraps it up)
You might want to simplify some of the things and characters in that final scene. I wasn't sure who the "collection of voices" represented and sometimes it wasn't clear who was talking. And when his character asks him "are you the imagination" I wasn't sure what that meant.
There's also some changes in tense and other grammatical aspects that you'd want to clean up (and maybe post this in a format people inclined to give grammatical help with could comment on).
Ultimately, I'd say this basically works but if you're inclined to revisit it, I think the climax / ending could use a re-write with more clarity about what entities are involved, what they represent and what the ultimate insight / resolution feels like.
If you're revising, you might also want to revisit the very beginning; if he's getting rejections that implies a finished story (I think? Was I right that he's trying to sell something he's written to a bookseller?) but the rest of the story seems to focus on him struggling to finish a story -- grappling with some kind of motivation issue or writer's block.
My confusion wasn't fatal but I feel like there's some advantage in having the opening scene clearly align with the rest of the story.