What Is Causing A Dark Ring Around My Mouth?

I have a dark ring around my mouth. It's been there a while. I've tried EVERYTHING! Went to dermatologists, tried bleaching creams, Vaseline, cocoa butter, EVERYTHING! And I've had no luck. It's embarrassing. People always making fun of me. They say it looks like a mustache. But there's NO hair! Any ideas of what I could do?