Lip colour going

Hi, noticed tiny spots slowly building up, seems like the pink in my lips is going, top lip middle part. I mentioned it to the doctor since I was unhappy about it. My doctor said it happens and there is no cure its normal. Now I seem to have a patch larger than a cm, it is obvious....even some light lip balms won't cover it. It has to be strong dark lipstick. I haven't worn lipstick over a year thinking that could be it but its worsened. So now I know its not that. Its on the actual lip.
What can I do about it?
What caused it?
Thank you.