The face and exposed part of arms are usually much darker than the unexposed part of the body. Reason for this is as these are constantly exposed to light and pollution. The skin also get darker as it is toughening itself against the environment so this is a protective mechanism which the skin is doing. Nobody wants their face dark so it is very important that you use sunscreens regularly. When you use your sunscreens a dramatic change is not going to happen within few days or weeks, the pigmentation or the constitutional tone which went darker on your face is a cumulative exposure over years so the sunscreen which is a protective mechanism by itself cannot give you all the improvement. In the night you should start some good vitamin C creams which is good skin lightening agent and very safe, you can start using it every night or every alternative night which helps in lightening your skin. Other safe creams to use are glycholic acid creams which exfoliates skin gently use them once in a week. Don't ever be harsh with the pigmented areas of the skin, when skin is already dark and sensitized to the sun if you try doing procedures like bleach, scrubbing very often the skin is going to go darker because that is how skin protects itself by becoming thicker and going darker, avoid it and be gentle with your exfoliation. Scrub and do your glycolic creams once a week. Eat healthy, eat lot of bright colored vegetables and fruits as they contain carotene's as well as bioflavonoid's these are natural anti oxidants and natural lightening agents. Vitamin C being one of the most important ones protecting your skin against the sun. If you still feel that after doing all these for 23 months also your skin is dark then you need expert help that's when you see your dermatologist