Hi There, C..
My daughter also hit herself, but she started a lot younger, like two and three years old. She'd use both hands and hit herself hard on the head, or she'd take one hand and keep hitting herself over and over on the side of the head. Sometimes, she'd even bang her head against the wall over and over and over. She'd also say she hated herself. That she was dumb and stupid. That no one liked her.
It wasn't until she was 5 that we (me, mostly) finally figured out she has Sensory Processing Disorder AND is a highly sensitive child. Double whammy! Some of her hitting herself was because she couldn't communicate well--she's been in speech since she was 3, and she's now 9. Most of it was her frustration at not knowing what to do with/how to express her feelings. We started taking her to a Childhood Specialist (a nicer way of saying a childhood psychiatrist) when she started Kindergarten, and she still goes. She loves her psych, and she's able to get a lot more ideas and strategies for coping than what her dad and I know.
We still battle with this, but she doesn't usually hit herself anymore or say she hates herself (although she still does the flight response, which I just hate). There is an issue with some low self-esteem, which we continuously work at.
My daughter is "years" above and beyond her playmates in terms of understanding, but she's still socially awkward when it comes to social situations. An example: If the teacher tells the class that she'll be gone the next day and that they'll have a substitute, she'll also tell the kids that she expects them to behave and be on their best behavior for the sub. Of course, kids being kids, the kids ignore their teacher and will talk and goof off and not listen and be anything but "good" for the sub. My daughter often comes home very upset, because the kids didn't do what the teacher said and "they should know better!" LOL I have a young adult trapped inside a 9-year-old body.
And that is what I deal with on a daily basis with her. Her trying to understand and assimilate and cope with why kids act the way they do, when she doesn't. She has a hard time understanding why they won't listen or they're not nice or they make fun of people (we're trying to learn about jokes and teasing and when it's done in fun or good taste or when it would be considered malicious. My daughter doesn't quite understand the concept of jokes and teasing). Again, that is all wound up and centers around both the processing disorder and the fact that she's highly sensitive.
I suspect I was also somewhat highly sensitive as a child, so I do understand some of the stuff she's talking about. But adding in the processing disorder--that makes it hard for me sometimes to be sympathetic and understanding, because it seems to take her forever sometimes to understand a concept I'm trying to teach her.
Any time a child hits herself, or says she doesn't like herself or she's dumb or stupid, or she should never have been born--it should ALWAYS be taken seriously. Children are more open than adults; a child wouldn't say or do something like that without a reason.
Taking my daughter to see a professional has helped our family enormously, and I also get some mom-psych time as well (because the psychiatrist needs to know how things are going at home--and, with Caitlin, there are some heavy metal toxicity issues and malabsorption issues and now some allergy issues--so she needs to know how things are going with me, since I'm my daughter's primary caretaker. My husband works full time, so I can stay home and work out of the house so I am available for her), which has helped me keep my sanity. So far. :)
Good luck.