Should I keep it in the fridge? Should I constantly discharge and recharge? Should I keep my hopes up or should I buy a new battery?
There is no hope for li-ion batteries in my experience, if they get old, they just get old, buy a new one, recycle the old one properly.
Li-ion batteries usually last a finite amount of time, regardless of use, though use will speed up the degradation process. Unfortunately it's just how it is. Read here for more on Li-ion batteries.