Clipboard manager. Set to record X selections too. That does it for me. Every bit of text I select is copied to clipboard, just below the value currently held there. In this case I'd have the full content of the docx file as a second item in my clipboard manager list with an email as a first one. I set the size of the clipboard items list to 500, just in case.

I couldn't live without some kind of clipboard manager. It was Ditto on Windows and klipper now on Linux. Dunno about Mac OS, but I bet there is something with similar functionality. It adds another layer of protection to Ctrl-Z everywhere and undo-tree-mode in Emacs. As a programmer I deal with text all the time and I need to be able to recover as much as possible if I make a mistake and this (clipboard manager + undo-tree + version control + normal undo) is the most robust system I could find.

Works for me. Across multiple vim sessions. Unless, you mean a new file thesis.txt, in which case the old file is untouched.