'the required index.xml file is missing'

Of course, just because you can recover the look of your document as a PDF file doesn't mean you can edit it in any reasonable way. But maybe....

Make a backup of the faulty Pages file. Get into its package contents, make a new folder on the desktop (for instance) and drag-copy the file "index.xml.gz" from the Contents window to the new folder. In that new folder, double-click the "index.xml.gz" to decompress it. If it fails, try a third-party utility such as unarchiver or Zipeg. You should get a file "index.xml". Trash the "index.xml.gz" that you just expanded. Right-click the "index.xml" that came out of it and choose "Create archive of". That makes "index.xml.zip". Rename it to "index.xml.gz" and drag that back into the original Contents window, confirming to replace the original "index.xml.gz". Close the Contents window and, with a lot of luck...

Another thought... if the creation of the new archive doesn't make it right, go through those steps, but instead of the right-click, Make archive, download and install "CleanArchiver. Then open CleanArchiver, set it to make a GZip, and drag the "index.xml" file from the new folder to the window of CleanArchiver. It should make a clean "index.xml.gz" in the new folder. Now use that to replace the "index.xml.gz" in the Contents window.

Regarding the comment: "...the same thing keeps happening and it loses the changes I've made between versions. That's the part I can't understand - I've been using this same method for years - why did it suddenly stop working?"

I don't think everyone has this problem often. Perhaps it is related to the number of saves applied to a document. I use Pages to create a half dozen new documents weekly, and make changes to about that many each week, so I suppose I am not a heavy user, but I have never had this issue. I found this post searching for a solution for someone else whose use habits I don't know well.

I guessed that lots of saves could lead to the issue for the same reason that a preference file gets corrupt -- from heavy use. Every time you change-save it you take a small chance that it will fail. Finally, your number comes up, to use the old idiom. The XML file serves as a type of roadmap for the document (more correctly called a bundle). Even Word documents are a type of bundle with no fewer than four files inside. Apple chose the XML format for Pages to allow better cross-application capabilities, while Microsoft has an older style that isn't much use outside of Word. Their new DOCX is similar to Apple's format that creates a bundle containing no less than ten files! We shall see if it is more or less durable than a Pages document.