In my fifties, fairly recently started self-harming.
I tried googling to get info to help understand why it feels like it helps. That's how I landed here. I read the first page of this thread (from 2013) and then jumped to this page.
There were times, maybe in my thirties, when I sometimes hit myself in the head with my fists. But once I hit my temple and it flashed white behind my closed eyes, and that scared me, so I didn't do anything like that for a long time.
Now I'm starting to really dig in the the baggage (severe neglect) and sometimes it's so hard! Sometimes I do the head-hitting, but more often punching the front of my thighs - I can hit so hard, and it hurts, but sadly no bruising - or the inside of my forearms, or now most often I dig my fingernails into my forearm, usually just pressing them in, but when it's really bad, I'll scratch (or more like gouge). Sometimes manage to do this one when the pain hits when other people are around. When alone, I have lately started to use implements - a fork, a butter knife - but that's kinda scary. Sometimes I slap myself really hard - feels more like actual punishment.
The other day I thought: I do it because it kind of short-circuits the real pain. The big inside pain stops while my body is experiencing this relatively little "outside" pain.
Today I thought: The physical pain feels like a step toward death, which is what I so badly want, when it gets bad.
So I guess there can be different reasons, or different effects, for any one person at different times.
My therapist doesn't know any of this. Even though I think the work we do is really good, and I trust him more than I've trusted anyone, but still.