I don't think writers are any more unstable ....
than those of other professions, as alluded to by the poster. For example, Hemingway committed suicide after receiving several electric shock treatments of which he wasn't apprised of beforehand. While he was a noted drinker of alcohol, from which his health issues may have stemmed, he didn't kill him self because he was high strung. Today, he would never be treated in such a manner.
Virginia Woolf was schizophrenic and had started to hear voices in her head just prior to her drowning. Agiain, modern treatment could have possibly saved her life.
These are only two off the lists provided, but I suspect there are more than a few who suffered health problems which today could be more successfully treated.
I've never read of Hemingway having cancer ....
I know he had had multiple electric shock treatments to the brain shortly before his death. Can you give me your source for claiming he had cancer? In 1962, I read as many of his obituaries as I could, but never read where he had cancer or was being treated for it.