Please help! bAD DIAPER RASH/FUNGAL

My baby has had a bad diaper rash 😢 i have a picture but idk if i should post it. Anyway i took him to the dr they prescribe him nystatin and it hasn’t help at all.. idk what to do i change my baby diaper every 15mins i check him constantly i had change diaper brand, wipes brand, i had air out his bottom, put desitin , butt paste, and vaseline. Nothing seems to help at all. I took him to the dr they said it was fungal and they still prescribed me nystatin and still no change at all. I have being using washcloths with water and aveeno soap. Still no change has being like this for over a month. Im at loss i feel like a bad mother idk what to do wish i can do anything to help my baby stop being in pain. Please help!