Check with your dentist and pediatrician. Maybe allergies or teeth hurt. We did psych meds with my son and it helped. Of course, that is a personal decision for you and your kid. They make helmets as well. Some kids out grow it, I think. Hang in there.

If redirecting her doesn’t work & if there is no other physical pain she is experiencing than I would seriously consider seeing a doctor who can get her a special helmet to protect her from self harming. It could be she is experiencing sensory overload & she may think she can hit the uncomfortable feelings away. I truly believe that when a child self harms then you have to make it very clear that this is not acceptable behavior & constantly & firmly tell her “No” every time it happens. (I should clarify to say I don’t mean for you to yell at her when saying no, just firmly tell her “No” as you take her head away from the wall & then try to soothe her with a loving hug & soft, soothing music. She is very young & from my experience with my own son who headbanged & hit himself in the head when he was a toddler it did work to stop the behavior. I hope I don’t come across as simplifying the issue, I wouldn’t do that since I understand it’s a very upsetting behavior that needs to be treated sensitively.

I agree that perhaps she has something dental going on. She may be trying to cause pain elsewhere to distract her from the pain in her mouth, if that is what she is experiencing. Like I mentioned in my reply to another question, I'd try redirecting her to something soft. Hugs :)