Well among a few other bugs and glitches, my game is now not saving. The little glitches like occasional lagging and freezing I can live with, even the occasional crash, but the game needs to save, otherwise it will get very annoying hah. What happens is I'll click the save button and it will start saving but then come up with a notice saying the game could not save (Error Code 12). This hasn't happened before, and I haven't downloaded any mods or content or anything in forever. Any help as to what might be going on would be much appreciated. Thanks!

Have you tried using your backup saves? See the Game Help Restoring backup Saves guide.

Also try using Save As rather than Save.