Aconthosis nigricans is diabetes related. My guess (no doctor) is that you have an insulin problem that is being missed because you are young and your body is doing the best it can. What you need is not the American diet but a life without out a lot of carbs. Carbs spike your insulin, increase appetite to want more carbs. It's a vicious cycle and the way most people eat these days is making it worse.

You have been made to feel bad and guilty but your parents were feeding you crap your whole life. Not their fault since people's food choices since the 50s-60s has been influenced by corporate profit. Your parents are wrong to harass you about your choices. How are their eating habits? Do they go out of their way to serve you healthy food when they invite you for dinner?

If you want to help yourself look into low carb lifestyle. That's how they treated diabetics before insulin was invested.