HELP PLZ!!!

This has been coming on my chest between my breast for 3 years now. My doctor says it's normal for big breast women and gave me some cream which clears it up some what but as soon as I stop using it it comes back. It looks so dirty but it's nothing but dead skin. It gets dry and flakes sometimes. Has anyone else dealt with this or know what it could be? I also have a dead skin patch on my chin also. I'm really starting to feel self conciliatory about this. Someone help!