There's a simple, not very helpful answer to that question. Blogging is just talking through my fingers, conversation at one remove. Writing is something else entirely.
We spent some time talking out more complex answers to that question. I'm going to take a crack at them here, with the proviso that I'll probably have to come back later and try again. Because even I don't understand this very well.
It's been true every time I've been on chemotherapy that I cannot write. It's not that my fingers can't touch the keyboard — they're doing that right now, clearly enough. Rather, something in my head fails.
I've said for years that I don't write like I talk. What I've meant by that is the part of my brain which produces fiction seems to run off an entirely different version of the English language. As if I speak two languages fluently, both of them English. I've long wondered if fMRI studies of writers deep in first draft mode would bear this out empirically. If you think about it, the process of learning to write well is rather akin to the process of learning to speak another language well.
The objective evidence of this assertion is available in most of my published fiction. Pick up almost anything I've ever written and read half a dozen pages. You'll find sentence structures, vocabulary choices, conceptual presentations and so forth that simply would not be present in spontaneous speech. Not even speech as annoyingly erudite and obscurantist as mine can sometimes be. (Or used to be, before chemo ate my brain.) I strongly suspect that some computational textual analysis on my blog corpus and my fiction corpus would suggest two different authors.
The issue isn't putting words and sentences together, per se. Regorafenib does in fact give me mild, transient aphasia, but that's just a bloody nuisance. I can still talk just fine, and except for the odd moments of aphasia or anomia, do not sound as if I am ill or confused. It's what those sentence are doing that matters.
Conversation, of which I consider blogging to be a special case, tends to be largely single-threaded with a fairly clear through line. You don't have to think deeply or terribly far ahead to function effectively. Note that my blog posts work this way. It also comes in brief chunks. A few sentences spoken at a time, or a few hundred words typed out over twenty minutes.
Fiction requires a much deeper integration of multiple aspects of story telling. Plot, character, setting, style, prosody, world building, continuity... the list goes on and on. Essentially, it's the same "hand of cards" theory I've often discussed here and elsewhere: [ jlake.com | LiveJournal ].
The number of cards you need in your hand for blogging is much smaller than the number of cards you need in your hand for writing fiction.
To abuse a metaphor, in terms of my writing faculties I've gone from playing high stakes poker at the pro tables to playing Old Maid with the kids on the back porch. This has removed me from the Producer role I've played and strongly enjoyed for years, and even compromised my Consumer role in that I can no longer effectively read books, either, because I can't keep track of that same set of complexities on the inbound side. For more discussion of this concept, see here: [ jlake.com | LiveJournal ].
The same problem applies to me working my chosen profession, from which I am now vacated on disability. I can write emails, memos and even meeting reports just fine, but I cannot handle the complexities of a hundred page business and technical requirements document, as well as the financial and legal issues inherent in drafting the associated contract.
Thanks to the cognitive impairments induced by chemotherapy and the physical and psychological stresses of terminal cancer, I can no longer do my job, either as an author or in my Day Jobbe career.
Which is to say, I cannot write. No matter how well I can still talk, I don't have the focus, continuity, or depth to write.
And this frustrates me to the core of my soul. It's part of the price I pay for remaining alive at this point, quite literally so. But a part of me is already gone, almost certainly beyond retrieval.
I am dying by degrees.