Based on your description, it appears that you have facial melanosis which could be melasma compounded by acanthosis nigricans or something similar. ​​These conditions have a strong relation to one's internal health and it needs to be evaluated as often quoted - Skin is the mirror of internal health. Your dermatologist will decide a treatment plan for you after a consultation which probably will include a retinol based night cream and a vitamin C based lotion. Sun protection is mandatory and will require usage of Sunscreens. You may also benefit from usage of a Laser device i.e. Q- switched N d YAG.