Please tell me how to treat itchy skin!

I'm a grown adult and my skin was very smooth when I was young except for acne on my face. But a few years ago, one area, especially my shoulder and upper buttocks, started to get really itchy. My skin started to get itchy.

ㅠ Because I kept scratching, my skin became dark and thick.. like atopic dermatitis ㅠㅠ But when I went to a big hospital, they said it wasn't atopic dermatitis. They prescribed me a topical medicine called Dermotason, but this only works when I apply it and doesn't solve the fundamental itching of my skin.. How can I completely cure itchy skin?