Your cold brew tastes like wet cardboard because you're leaving the grounds in overnight
I let my coarse grounds steep 14 hours in the fridge once and it came out bitter and gross, so now I cap it at 12 and it's smooth every time. Anyone else track their steep time or do you just eyeball it?
12 hours is not some magic number. Plenty of people steep 16 to 24 hours and get great cold brew. The real problem is grind size and ratio, not the clock. If your batch went bitter after 14 hours, your grounds were probably too fine or you used way too much coffee for the water. I have done 18 hour steeps that tasted smooth as anything. Time is just one thing in the mix, not the whole story.