I was hitting myself in the side of my head as hard as possible, sometimes not as hard, dozens of times. I didn't notice it until after a lot of hits. It always leave the side of my head feeling sore and bruised every time. It's usually sore even a week after.

I wouldn't hit the front of your head. That's your frontal lobe, the most advanced part of your brain. You might end up a zombie if you do that too many times. That's the only part of my brain that I cherish. Some parts of the brain is primitive, mostly the back part of the brain.