Why are the inside of my butt cheeks, inner thighs and armpits dark?

I'm black with Ethiopian and Tanzanian. I have always had dark underarms and also dark inner thighs (although i know this is because my legs rub together when i walk). But I've recently noticed that it's really dark between my butt cheeks.. like almost black. This makes me insecure and scared of getting intimate with anyone!! What do you recommend to get rid of this and does my ethnicity matter or come into it? Thank you I'd appreciate any answers!!