While it is always impossible to tell for certain without doing a physician exam and actually seeing your skin, your question is almost straight from a text book: type 2 diabetics, who are resistant to insulin, develop areas of darkened skin that look "dirty." No matter how much you wash the areas, they won't seem to lighten, because your skin is not dirty. Instead, for reasons that aren't entirely clear but probably have something to do with the growth factors that are changed and affected with insulin resistance, your skin has become darker in these areas. This disorder is called acanthosis nigricans, and will often develop in the folds of skin in those who will become diabetics or already are. It can also be associated with some malignancies, and so any persistent skin changes should be followed up with a physician. It would also be important to be working closely with a physician to keep your diabetes in check. While the changes to your skin might not hurt you in the long run, a lifetime of poorly controlled diabetes can be devastating to your heart, kidneys, eyes, and other organs. I would definitely advise making an appointment with a primary care doctor
to review your health history and make appropriate changes to improve your health.