Dark marks on skin health problems?

Hi, I have, since childhood, had dark skin patches on my abdomen and lower back. Over the last few years they've nearly tripled in size and I've been concerned for a while about them. It seems like a bunch of layer of skin over top of another, kind of like it's dirt.

I went to a hospital and they said I was producing too much insulin and that was the cause. The medicine they prescribed was metformin and I've had nothing but problems with that medicine. I stopped taking it and the marks have only gotten worse. I was wondering if anyone had any ideas as to what I could do to try and combat this, aside from taking metformin again. Thanks.