He doesn't know.
My partner doesn't know I self-harm. He knows that I manically pull my hair out to make myself feel better (Trichotillomania). But he doesn't know I have urges to cut. I restrain them as well as possible, but it drives me insane to look at my skin and know I could. He used to injure himself in a similar way and is on the path to recovery as I spiral towards the urge to do it myself.
I can't tell him. One of the things I overheard him say to another person about me was 'Don't ever let him figure it out.' in regard to me potentially hurting myself for relief from my own personal panic and fear issues.
F*** I need a therapist.