I'm thinking that it has to do with the feature that tells you when someone else has posted, since you read the thread----and since you said it took you several hours to type it, that seems likely. When you hit "Submit", it will say, something like, "Someone else has posted since you clicked on this thread, do you still want to post". It appears on the same page as the message: "Your post has been submitted successfully" (or whatever). If you don't scroll-down to read what it actually says, and just assume it says it was successful, and close the window, your post will be lost, forever. ALWAYS, right-click and "copy", before clicking on "Submit".
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White female; age 59; diagnosed Aspie.
I use caps for emphasis----I'm NOT angry or shouting. I use caps like others use italics, underline, or bold.
"What we know is a drop; what we don't know, is an ocean." (Sir Isaac Newton)