Hey Gwenny, I'm a writer as well and work is what kept me from stopping before. My most recent gigs have been
PR/marketing/copywriting type work, and even that I found incredibly difficult in early sobriety. I often work in tech so frequently the job involves learning about something technical and translating it into copy. My brain would just absolutely jam on that combination of precision and flow. Usually about 10 days after quitting I would just break down and have a drink because I felt like I couldn't do my job.
This time I took advantage of a lay off to quit, so I can't put my finger on just when it came back. But I know that after the first month I was working on my personal writing projects again without feeling like my brain had rusted through. Now at three months it still comes and goes. There are some days where I feel just illiterate. I'm freelancing so unfortunately deadlines are a reality for me too... and sometimes I am just subpar. It's a bit easier for me in my line of work... If some of this copy is a little weak, it's not that big of a deal. As long as it gets the job done. Your project sounds more demanding and also more precise. But there is something to be said for recognizing the ceiling and not beating your head against it... whether that's renegotiating the deadline, as BeMyself suggested, or just dialing down your inner perfectionist and accepting work that's just OK.
As for vitamins, are you also taking fish oil? I take fish oil twice daily. It's highly recommended to anyone with brain injuries of any kind (which I've decided includes me, at least in some senses). I also take, along with my multi, B Complex, zinc/magnesium/calcium, and a mega dose of C. Speak with your doctor if you've got one, etc, but definitely investigate which vitamins you may personally be lacking in. I noticed the improvement when I began taking them and these days if I forget for a couple of days I feel sluggish.