Answer:
Long and slinky like a trout, never sings till it's guts come out.
No matter how little or how much you use me, you change me every month.
Slayer of regrets, old and new, sought by many, found by few.
I cannot be felt, seen or touched. Yet I can be found in everybody. My existence is always in debate. Yet I have my own style of music.
Many hear me, but no one sees me, and I only speak when spoken to
I appear in the morning. But am always there. You can never see me. Though I am everywhere. By night I am gone, though I sometimes never was. Nothing can defeat me. But I am easily gone.