Rash between boobs?!?! HELP 😭

Hey, everyone! 
So I'm 20 and this has been a problem for me for years, since high school, and I never knew what to do about it or fix it. Under my boobs there's a rash like where the underwire of a bra would be, and I wanna know if any of yall have experienced this or have any suggestions to get rid of it! It makes me really embarrassed and I wanna get my confidence back!