"The 'not being me' thing? I wasn't paying attention, I think it was when it effectively broke. Lemme test something."

Instead of a horn, a fingernail! His left thumb's fingernail, to be specific. As it grows, he goes and grabs a nail clipper. First, he reshapes the fingernail a bit, to demonstrate that he can. Then he clips it halfway. Can still reshape it, can mend it. He clips it so it's only barely still connected to him, and it's still him. He clips the rest of the way, and that's it.