Irregular Periods

Okay, so I'm on my 3rd period this month meaning less than a full week in between the bleeding and I'm losing my mind. I talked to my mom about it because I want to get on birth control so it would regulate and she thinks "I'm sick" and need to go to the gynecologist. I don't feel any type of sick or illness other than the usual side effects of periods. She doesn't want me on birth control because of side effects like cancer but I want to be regular. I'm 17 and feel like it should be my decision. After all I could have gone behind her back and gotten birth control at a free clinic since I'm underage, but I'm choosing to be open and honest and she is not compromising. I don't know what to do because I'm sick of constant periods but she already made me an appointment with a PA at her gynecologist's next week.