Self mutilation is often used as a cry for attention. I'm not saying this to speak badly of you, so please don't take it the wrong way. I used to cut myself, and now I have an eating disorder I am working on. So I am in no position to judge.
I read what you just wrote about your parents, how your step mom doesn't want to pay for therapy because it will cut into her shopping fund. Maybe you feel like you're not getting the attention you deserve because of the step mom? Did you start hurting yourself around the same time she came into the picture?
Another reason people hurt themselves is for the control. Do you feel like there aren't many things in your life you are in control of?
I struggled with hurting myself for years, I might be able to help if you would like.