When i told mine last week ( I am DID/MPD so we were talking about that too), I just said "I am afraid of things that hurt me, but - this other part - does this" and showed her the scars. first she saw only three and looked for a moment like she would simply pass it on, so i moved the clothes so she could see more. She never changed her demeanor, but looked at them carefully and told me she was glad i had brought it up because she had thought something like this might be happening.

I didn't have any emotion when i told her, i didn't try to down play it. Just gave the bare facts and then answered her questions honestly about how often it is done, how long it has been going on, and if i had told my T.
Best of luck, Kiya