... so maybe I'm hallucinating.
Perhaps not Clod. I remember following the
via link in the Lester Letter thread opening post that BigV cited. It leads to the
Letters of Note website which has the Lester Letter (titled
Servant until Death) and also contains your
Deep Sickness Seized Me letter.
On the left side of the page is a list of the website's most read letters as links and I've scrolled through them before. At that time, late 2009, I think a brief description appeared as the cursor highlighted each one. IIRC, the link to your letter was on that list and looked something like:
Deep Sickness Seized Me - first hand account of Lucy Thurston's unanaesthetised mastectomy. The descriptions no longer appear even though there's a plus sign (+) preceding each title. That's a shame since the titles can be ambiguous.
I found the letter interesting enough; but, decided against posting it in The Cellar. I didn't think it would be well received back then when Trilby was still going through turmoil from her experience, monster had a friend affected, and Sundae's mom did too. It may have seemed insensitive while those conversations were taking place.
Perhaps someone else posted it. Perhaps they had second thoughts and deleted it. There's a way to search for links and attachments which I did (cursory searches); but, I didn't find it either. That doesn't mean your original encounter with it didn't stem from The Cellar.