Answer:
When I’m metal or wood, I help you get home. When I’m flesh and I’m blood, In the darkness I roam.
No matter how little or how much you use me, you change me every month.
Long and slinky like a trout, never sings till it's guts come out.
I can ruin your day, or darken your life. And while I'm expected, I'm never alright. I'm bad for business, and worse for your hair. Wherever I am, there's something not there.
As round as an apple. As deep as a cup. All the king's horses can't pull it up.
If it’s information you seek, come and see me. If it’s pairs of letters you need, I have consecutively three. What am I?