Hi..
Welcome to HEALTHCARE MAGIC ..
I have gone through your query and can understand your concerns..
As per your query there can be a few common reasons for skin darkening like Excessive sun exposure, anemia or 
iron deficiency, stress, allergies or side effects to certain medication, post inflammatory 
hyperpigmentation etc..
I would suggest you to 
consult a Hematologist and get evaluated and he can advise you appropriate doses of 
Iron supplements for relief from iron deficiency..
You should also take ample of sleep as well as relieve stress with meditation and breathing exercises..
You can start depigmentation treatment like wash your face with 
Salicylic acid and glycolic acid containing facewash like AHA glow, Melaglow etc..
Appe Kojic Acid containing creams..
Creams or ointment containing Azelic acid, arbutin and Vitamin E can relieve..
Exfoliate your face with a gentle scrub weekly..
Drink plenty of water, take Vitamin C supplements and a nutritious diet..
Hope this information helps..
Thanks and regards.
Dr.Honey Nandwani Arora.