You are trying to remove the entire light fixture. If you hold the meta can you can twist the glass counter clockwise and the glass comes off. If you cant get the glass to turn if you continue to pull down the fixture is held in with two spring arms that will eventually come out of the ceiling. Don't be shy you won't break it. When you install the new bulb do not touch it with your fingers as the oil from your fingers will cause premature death for the bulb.
Brian, Loretta & Daisy (Golden Retriever)
2008 Holiday Rambler Endeavor 40 PDQ , ISL 400
2014 Ford Explorer toad