15 years must feel like a long time at the age of 26, more than half your life. Your brain must be very used to and dependent on getting the rush of chemicals that are produced from harming yourself.
If it's not too personal can you recall what made you start? 11 is about the time we start high school in the UK, I remember that being a stressful time transitioning to such a big school, new people and teachers etc.
I never experienced bullying until I reached high school, I remember infant and junior school being more or less ok, I didn't have many friends but the few I had were good ones.