For a while now I have been convinced that I have bipolar disorder or possibly borderline personality disorder. I feel that they are the only possible explanations for my mood and behaviour. I would like a professional opinion, but I'm only 16. Is there any way that I can speak to my doctor without my parents finding out? Also, I have been diagnosed with social anxiety disorder in the past and I'm nervous about speaking to a doctor. Is there anything I can do to feel less anxious about the appointment?

Doctor's are bound by confidentiality, so I don't think they're allowed to tell your parents anything about your health without your say so.

Here's an excerpt from the General Medical Council guidelines:

42. Respecting patient confidentiality is an essential part of good care; this applies when the patient is a child or young person as well as when the patient is an adult. Without the trust that confidentiality brings, children and young people might not seek medical care and advice, or they might not tell you all the facts needed to provide good care.

As for not feeling anxious about your appointment, I think a lot comes down to the doctor you visit. I've seen patients with pretty serious conditions laughing away because of the personality of their doctor.