Answer:
Always old, sometimes new. Never sad, sometimes blue. Never empty, sometimes full. Never pushes, always pulls.
I know a word of letters three. Add two, and fewer there will be!
Always wax, yet always wane: I melt, succumbed to the flame. Lighting darkness, with fate unblest, I soon devolve to shapeless mess.
I cost no money to use, or conscious effort to take part of. And as far as you can see, there is nothing to me. But without me, you are dead.
A type of hammer that brings a room to order.
I’m grown from darkness but shine with a pale light. Very round I am and always a lady's delight.