We have an 8-year-old alter in our system who banged her head with her fists when she felt misunderstood, unheard, dismissed, disrespected, or out of control. She was really hurt by a therapist who used to have her spend the night, and she got triggered. We reported that therapist to the board, and we left that therapist, but she was really frustrated with being triggered by the therapist. She didn't bang her head to punish herself per se; she did it as a means of frustration and communication. She wasn't that verbal on the outside either, so she would be more emotional.

Flash forward over a decade later, and our current T is really kind toward all of our parts, including the littles. We told her about our 8-year-old little who used to bang her head, and she asked if there were "helpers" inside who can help our little when she feels upset. The helping alters inside started helping the little not do that, and they would comfort her and help her through the coping skills we all learned - separately at first, but now more together now.

In terms of singletons (those without DID alters/parts), it could be the "inner child" in you who is frustrated and hurt. Most children experiencing trauma tend to blame themselves, and sometimes that gets externalized in the form of self-injury. When we are subdued from expressing our emotions, and when the trauma is so high, children may act out by banging their heads, etc. Over time, if this remains untreated, then adults can revert back to this maladaptive coping whenever they feel triggered in similar ways. Other times, mental illness in adulthood might bring about a kind of physical urge to self-injure, such as those with bipolar. If you are struggling with comorbid disorders like PTSD and bipolar, then the likelihood of self-injury increases.

Many different treatments for self-injury, including head banging, exist. Some include CBT and DBT. Others will include things like finding an alternative outlet to release internal pain, such as screaming into a pillow, doing artwork, journaling, ripping up a phone book (if they even have those things anymore, LOL), etc. As long as it's not further harming yourself. Some T's will disagree with using ice or a rubber band on the wrist to snap in lieu of sledding or other forms of self-injury, because that still reinforced self-harm, which is not healing. Sometimes other psychotherapy methods like person-centered treatments or other eclectic psychotherapies might help, such as figuring out why you self-blame, and why you punish yourself, and where those negative thoughts came from, and what your triggers are when you hear that. Then, you can redirect those negative internal messages to seeing them for what they are - non-truths. It's not your fault. And it's okay to make mistakes. Many affirmations that counter those negative thoughts, after figuring out first where they came from. Perhaps some trauma processing and finally getting at the painful memories that preceded your self-injury. For my DID system, it takes a combination of efforts to help treat our individual parts as well as us/me collectively as a whole.

If you have a therapist, discuss this particular self-injury with your T. Find out what treatments your T can offer to you.
See what others who know DBT have done to help them (I don't know DBT very much, other than I know it doesn't work for me and is a trigger to me, but I heard it helps a lot of people)
See what others who know CBT have done to help them (I do have fond memories of CBT treatments, when I was in a trauma facility; I can share them in a separate response here if you'd like).
And see what others have done to help with their unique self-injuries.

If your self-injury seems "automatic," one way to prevent yourself from harm when you feel like that is to put on a beanie or hat, and then associate that hat as an object you care about. You care about the hat, and you don't want to hurt it. It's one way to learn to protect your own body through a mediator like a hat. (This is just something that one of my inside parts came up with just now, so it's not from a therapist, mind you - it's just an idea.) Have your safety hat ready for when you start to feel anger and self-blame coming on. Pause, get the hat, put it on, and focus on the care of the hat. If you're frustrated with yourself, write in a journal or online about what you're feeling, and how the hat is currently protecting you from self-injury. When you feel those emotions and stuff all bottled up inside and needing an outlet, you will find many healthier ways to let those feelings out - either by crying into a pillow (with your hat on), crying with a friend who can safely comfort you, writing things down and placing them into a container until you see your T next (pacing and containing), using a CBT guide to address the trigger, the automatic thoughts, the emotions/feelings from those automatic thoughts and triggers, the relapse behaviors (or in this case, maladaptive coping like self-injury), the reinforcements for self-injury (these are your perceived benefits from self-injury, such as releasing frustration and pain), the NEGATIVE CONSEQUENCES if you continue in the relapse behavior (these are negative things like you having a concussion or shortening your lifespan or feeling more pain and headaches, etc., when you self-injury), and, finally, the ADAPTIVE COPING RESPONSES (ACRs) you can use to replace the reinforcers and relapse behavior, such as putting on a safety hat and finding healthier reinforcements to let out frustration and pain, such as coming online and venting, learning proactive coping skills when faced with a challenge, taking time to pause, grieving over the losses experienced in childhood when you never got comforted and therefore learned to self-blame and later on self-injure, etc. It's a deep process to use various forms of CBT treatments, like the one I briefly outlined (which is what I learned in a trauma treatment facility years ago). Many different therapists will use different forms of CBT to help you.

Hopefully one of these suggestions are worth trying. If not, I hope you find the help you need to cope with this. (((safe thoughts, wishes, and hugs)))