2+ weeks long

I'm on day 16 of my period. I have the implant in my arm, but my periods have been crazy heavy (average 10 super tampons a day) and lasting 2-4 weeks since I was 14. I'm currently on day 16 with an extremely heavy flow. What would cause a period to be this long?? Doctors haven't found anything other than "I'm unlucky". So fed up.