I think you'd probably be best off seeing a therapist or a psychiatrist. They'll be able to help you, find out your triggers, when it started, events in your life and maybe another alternative to pulling out your hair.
The GP would probably just recommend the same. It's more common than you think it is. I know someone who pulls out her eyelashes and eyebrows. Don't let it beat you down. You're 15 and you'll get through it ❤️