Hi, So I'm Now 19 And Ever Since I Was

Hi, so I'm now 19 and ever since I was 12 I began to get really dry patchy skin under my arm pits and it was discolored. I think it might of started by using this bronzing lotion that helped you tan. Growing up it spread onto my stomach and now my stomach is discolerd and super dry and scaly from the rest of my body. What I do to make it look better is scrub off all the dry skin with a microdermabrasion treatment, it takes off all the dead and dry skin for like a few days and now my skin just has the discoloration. It usually comes back right after a few days and I moisturize like 3 times a day too. Also tanning does help cover it and make my skin look normal and it's not as dry but when the tan starts going away it looks really bad again, but tanning was probably the best option that I've done. I have gone to a doctor for it but they don't know what it is.