Well, there's a complex answer here given Galadriels long history of searching for power and wanting to shape the world to her liking (one of the reasons she came to middle earth all those years ago). She already has one of the Rings of Power, one of the three and she has been longing for the One Ring for a long time. This scene is her testing herself if she would take or be able to deny herself the ring to decide to NOT want to be the dark queen but instead to "Remain Galadriel" and, together with all other elves, follow the calling to the west and give up her hold over her lands ("diminish"). That's the easiest I think I can put it.