You said your coffee tastes flat and stale now, but is it really that big of a deal? Like, on a busy morning when you're half asleep, are you actually noticing those tiny details? I've used pre-ground stuff for weeks and barely tasted a difference, especially after I added some creamer. Sometimes I wonder if we just convince ourselves it's worse because we read it should be.
My buddy Mark swore he couldn't tell either, until his grinder broke and he had to use month-old grounds. Even with creamer, he said it tasted like wet cardboard and finally got what I meant, @grant540.
Read a study once where they gave people the same coffee but told half of them it was cheap stuff. The ones who thought it was cheap rated it way worse, even though it was the exact same brew. Makes you wonder how much is in our heads, like grant540 said. But then jamie_taylor's friend with the old grounds proves sometimes the drop in quality is real and you can't ignore it. Maybe it's both, you know? Your brain can trick you, but truly bad coffee will cut through the creamer every time.