Your step-father is not only wrong, he is emotionally abusive. You are on a very dangerous path and need to get help
for your eating disorder. Yes, that is what it is. You are showing anorexic symptoms and it will only get worse. Talk to a counselor at school, your doctor, or another trusted adult and have them refer you to a therapist experienced with eating disorders. Your ED is related to how you feel about yourself and you need help working through both so you can go on to live a good life.