Today I finally reached three whole years without self-injuring. I had been self-injurying in one way or another since I was around 15 years old. I took the big step to razor blades when I was 16 and it was a downhill spiral ever since. I didn't appreciate or understand the addictive nature of the behaviour until many years later, and by then I suppose it was too late.
I still think about it every day. I still fight the urge to hurt myself every single day of my life. I dream about it, fantasise about it, even make up plans of how to do it and get away with it (completely impossible now that I am living with my partner, by the way). I actually sometimes wonder if people with addictions to drugs or alcohol think about their given up addictions in the same way that I do with self-injury.
I see people at least every day who have harmed themselves. Some are friends, some are strangers. Every time I see a young person with fresh cuts on their arm when their armband slips or their sleeve falls down I feel a pang of worry for them. I worry when I see young people who have cut themselves because I too was once them and what started as a scratches soon developed into so much more. I worry that they don't appreciate the highly addictive nature of the habit, and that some day they will be struggling to give it up like I am.
Self-injury is the act of intentionally causing yourself physical injury by cutting, burning, interfering with wound healing, hair-pulling (trichotillomania), skin-picking (dermatillomania), head or wrist banging... and many, many other ways. The reasons that people self-injure are as wide and varied as the self-injury methods. People self-injury to cope with overwhelming emotions, stop/prevent/induce dissociation, as a physical representation of emotional pain, inducing euphoric feelings, gaining control, self-punishment and many other reasons.
People that self-injure often keep it hidden from family and friends because they are ashamed. You could pass people in the street and never know whether or not they self-harm.
For more information about self-injury you can visit Self-Injury: A Struggle, which not only has an excellent FAQ about self-injury (for both people who self-injure and their friends and family) but also a large and tight-knit forum community where you can talk to others and receive support.