The depiction here is actually a bit different between the original book and the movie. The part where Frodo sees a vision of Galadriel as the owner of the One Ring is the same, but in the novel, it's not that scary and she doesn't struggle that much to shake it off. She just looked like a greater and more beautiful being, but the vision vanished in an instant and she refused with a smile. However, the actual reason for the refusal is the same in both the book and the movie. She would use the One Ring, kick the absolute shit out of Sauron and his goons first, and then become a Queen that everyone loves and fears. Basically, she’s saying 'don't give it to me' because she'd turn into a toxic, obsessive 'menhera' type.