She closes her eyes for this. The skin and muscle of her forehead start shifting visibly, two circular depressions appearing where they get thinner and thinner. She frowns a bit, but says "Ow!" when that pulls something she's working on and stops frowning. Eventually the shifts settle and there are two very clear discs on her forehead, but no horns are forthcoming.

"...I don't think I can do it."