Should I get the Mirena?

Hi there,
My name is Camberley, I am 16 years old going on 17 in December. I have been in a relationship with my boyfriend for nearly 6 months we are very serious, and lately we have been talking about sex and contraception. Me and my boyfriend went to the doctor and I was put on the Mini pill, Noriday as I suffer from migraines. I have been on it for over 2 months but I have gained a bit of weight, my period is still very heavy and I have been feeling a bit down.. I have been looking for other options of contraception but I have a big phobia of needles or anything going into my arm. So that wipes out the injection and the implant. I have really heavy periods and bad cramps and I came across the Mirena IUD. I am very interested in getting it, I am aware that it'll cost more or less $500 New Zealand dollars (Including all the doctors appointments) So I looked into getting it. My boyfriend called up the doctors and booked an appointment they didn't seem to sure on giving me it as I am only 16 and haven't had any children. How is this exactly a problem? Will it hurt more? I'm not really worried about the pain! I just want a reliable form of contraception as I'm terrible and remembering to take the pill!! I was just wondering if anyone could help me out and tell me about it, how it works, does it hurt getting it in, if you have got it before having any children?? Thank you so much!