22 Male,I have itchy dark skin on my both butt cracks for over 3 months

Hello,For over 3 months I have a dark skin on my butt cracks and they are itchy.The itch is worse at nights.I visited 3 dermatologist and I used different sprays,creams,pills etc by their suggestions.Nothing worked.What do you think they can be ?

I have photos too.But I do not know the rules so i didnt share them.