This is what you should do
I'm serious on this one, I'm not calling you crazy but maybe you should go to a physiologist. Stabbing yourself with a pencil just doesn't sound normal and I'm sure you know it. Sure, it could've been caused by rage and rage can make people do crazy things but this is just over the top, if you know where I'm coming from which I'm sure you do. While I was reading, schizophrenia came in mind especially when you said that you would completely forget about what you've done afterwards which is a symptom of it. I'd recommend you to go see a doctor before you try to help yourself, on your own. Your rage could be triggered by a chemical imbalance in your brain also caused by schizophrenia.
I'm serious. Go to your doctor as soon as possible. If you avoid this one day you could do the unthinkable and do something you might regret. You wouldn't want this to happen, now would you?
Think about this for a while and I'll give you a couple of sites to help you out. Also, you can always ask me for advice. I'm always happy to help someone out, especially in your situation.
Good Luck Dear.
Health Scout - Schizophrenia
Health Scout two
A website especially made for Schizophrenia
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Jump on it!