I began to SI when I was 15. It was due to lack of control in my life. I had undiagnosed OCD at the time (which consumed a lot of time thinking and obsessing over little details), and also undiagnosed ADHD (so when I wasn't obsessing, I was running all over the place trying to keep up with all the tasks of daily living). It was a mess.

Then I started therapy, started working with this amazing therapist, and things are a lot easier to control. I dont SI anymore, I am doing well in school, life is a lot calmer. I suggest that you get a therapist and start treatment.