How can I lighten the dark patches on my skin? I’m 18 but parts of my skin including arm neck and knees have darkened.

When I was younger all my skin was a light brown colour. a couple of years ago patches have turned a lot darker. This includes my neck, mid and lower arm, knees. I visited a clinic who advised me to buy a serum exfoliate and spf. despite using this religiously they haven’t worked. I need a good dermatologist who will be able to lighten my skin. Many people have stated that my neck looks burnt. I’ve included pics of arms but it’s a lot worse in person. I want same colour as hands.