How To Get Rid Of Dark Skin Under Armpits And Inner Thighs?

All my life i have had darker skin under my arm pits and inner thighs and hate it!! I have went to a dermatologist and all they gave me was cream that Did NOT work..I'm 40 and yes all my life i was and still over weight I'm in great health over all no diabetic and my B/P is great. What can you do to help me to take this way? Thanks Patty