Answer:
Lovely and round, I shine with pale light, Grown in the darkness, a ladyβs delight.
Kings and queens may cling to power and the jesters may have their call. I am the most common but I can rule them all. What am I?
I can be sweet or sour. I do not rhyme with any other word
There is one in every corner and two in every room.
I never was but always will be. No one ever saw me but everyone knows I exist. I give people the motivation to better themselves everyday
No matter how little or how much you use me, you change me every month.