It is not uncommon to get yeast rash under pendulous breasts where very little air gets at the skin. Nystatin powder or cream is very effective. You should have relief after even the first application if that is what is going on for you.
Candidal intertrigo usually occurs in the flexure areas of the body such as the armpits,the groin area and beneath the breasts. This is usually associated with an underlying chronic illness such as diabetes ( have you been assessed along this line?).
I suggest you push through with your physician consultation in case your medications may need to be changed or modified. An assessment of an underlying systemic disease like diabetes may also be needed.
Did I say I am called a pre-diabetic? That is so intresting. I am on meds for that. Thankyou!