I understand your health concern.
The cause of this is a fungal infection
in medical terms. You have had a past infection in your younger ages and it has continued to the present state.
The mere dark complexion suggests it is a fungal infection.
Follow these measures:
1. Dry yourself thoroughly after bathing. Then only put on your clothes.
2. Use a simple antifungal cream like clotrimazole
on the darker areas on a routine basis.
You should feel better soon though 100% cure is not possible.
Wish you good health.