At 18, you don’t need your family to listen to you or to believe you. The only person who has to believe that you need and deserve treatment is you. Instead of spending your energy on arguing with your family, explore how to get the treatment you need. Instead of channeling your pain into self-injury, get help to feel better.
Since you were diagnosed, I’m assuming you saw a clinician who is qualified to do it. That person can give you a treatment plan. If you can’t afford to follow up with an individual therapist, there are alternatives. You may find it helpful to take a look at this article.
Taking charge is likely to make you feel better. Doing the hard work of therapy now while you are in your teens will set you on a path for a happy and productive adulthood.
I wish you well.