How to ask my parents for mental health treatment?

I have been suffering from bipolar disorder and anxiety for almost 2 years (I am 17 now) and I have had to deal with it all by myself as I have never expressed my true feelings to anyone yet, but lately it has gotten so bad that I cannot focus on any of my schoolwork and simply I feel horrible. I have tried asking my mother in the past if I could see a doctor or a counselor to talk about my problems but she replied back with "its probably just PMS". I'm not sure how to confront my parents again about this issue. please help.