How To Get Rid Of Spreading Itchy Patches Between Breasts?

I have had a slowly growing rash for the last few years now, started as a small discolored spot between my breasts (cleavage area), then eventually spread to up between my breasts and heading toward my belly, it s making me uncomfortable, in warmer weather is gets hot and itchy, but most days it s just darker than my skin tone and leaves dry patches. Help?