I had the same problem, so I tried derma rolling (you can buy one on ebay for 5$), and then I applied something called "life flo health Retinol-A, 1% roll on". I've tried a few retinol creams but I didn't like them, got red and dry and itchy all over my face for weeks after. So i found this roll on serum thingy from Iherb, it was so cheap to! I just wash my face as usual with my neutrogena acne wash, roll quickly over my face with the derma roller and focus on the problem aereas, then I apply a serum or cream, and finish off with rolling the retinol over the aereas with hyperpigmentation. My skin looks exactly like it did before i had acne now, and it looks so healthy again yay

Vitamin A is definitely a product that clinically proven to reduce dark spots. But it's super powerful and can be irritating to skin (causes inflammation, redness, flakiness, stinging, etc.)

So the next best bet is using serums that contain Vitamin C or Niacinamide. (But don't use both at the same time, because they are both active ingredients and can reduce the efficacy.)

Here's a list of products that should help you! <3 Remember it won't be magic and disappear overnight. You have to be patient and consistent with using good quality products.