cannot download my own sims from the gallery

@KricketKorner  The issue might be that you have too many Tray files, or that only some of them are corrupt.  You could test the files if you wanted to salvage at least some of them.  Each in-game Library item corresponds to a group of at least four Tray files, all with exactly the same timestamp.  So if you view the Tray folder in detail mode and sort by modified date, you'll be able to distinguish among different sets of files.

You can test the files in batches, setting aside any that work and splitting and retesting that don't.  As long as you keep all the files from each library entry together, you should be fine.  You might also find that each set of files works on its own, but the entire group causes problems due to its size.  In that case, if you don't want to delete any items, you can set aside some of them for later use.  Tray files can be swapped back and forth like any others.