I think it's probably because of your age and the balance of your hormones. I was similar in my teens and my uneven skintone improved with age.
You should probably ensure you have a good skincare regime. Dermalogica have a good range of products for problem skin, however, they are quite expensive, but do last long. You can purchase them at certain beauty salons. They reccommend you have a "skin mapping" consultation, so they can select the correct products for your skin type.
Apart from that, a healthy diet and drinking plenty of water, as you already do, should help.