How to download your entire posting history from day 1:
Click your avatar at upper right on any page. Click the person icon at the bottom right of the popup. That brings up your profile page. Click “Preferences” at the right of the upper row of menu choices. That should auto-select the leftmost choice on the second row of menu choices: “Account”. If “Account” isn’t selected, click it. Now you’re on the right page.
Scroll down and near the bottom you’ll see a heading “Export your data” with a button just below labeled [Request archive]. Click that button. An “Are you sure?” popup will appear. Click [Yes]. Another popup will appear saying “we’re working on it”. Click [OK] to dismiss that.
That’s almost all you have to do.
The system will do an export of all your work, make it into a zipfile, then notify you when it’s ready. For my pretty hefty corpus of work, that took about a minute then a DM appeared in my notifications. The DM is from “system” and is titled “[User Archive] Data export is complete”. Click that to open the DM. Which has a link to click to download your archive to your PC.
The zip file contains several files, most of which aren’t important to you. The big one is all your posts. The format of that file isn’t too friendly to a non-IT person, but even if you can’t make it all pretty, you can at least find all your prose organized by date.