I make my own fabric softener “sheets” by mixing 2 tablespoons of inexpensive liquid fabric softener with 2 cups of water.
I cut up my old dishrags and soak a small square in the mixture. Then I squeeze out what I can and throw it in the dryer.
For a while, I was having a problem with mildew forming in my unused fabric softener mix. After I started adding 2 tablespoons of vinegar to my mixture, I have not had any more problems with mildew.