Really dark skin on each side of my neck and abdomen?

K so when I was younger I had really a horrible rash all over my neck and lower abdomen that I only recently found a way to clear up butt the problem now is that this rash has left very dark spot on my neck and abdomen these spots are all the way down each side of my neck and in a large circle on my abdomen (my skin is about the color of milk chocolate these spots are 2-3 shades darker than that) I want to lighten these spots but I don't know how if anyone has ay ideas they would be appreciated.