Does your school have a counselor you can talk with? One you can tell your story to, including how your parents are reacting and tainting your GP's opinion. If there isn't a counselor, I would suggest looking for a place that offers mental health services in your community.
It is difficult for people to understand mental illness. As an adult, I was constantly challenged by my siblings... "why are you still taking medications", "why are still seeing a therapist", "you need to change therapists", ... They didn't get it until I was put on suicide watch during a hospitalization and had to undergo ECT treatments to get me out of that dark place.
Depression is misunderstood. It is the 9th leading cause of death, but isn't studied like other illnesses. That needs to change. People need to be educated as well.
I wish you well in your pursuit of help. Keep looking until you find someone.