The suggestions about tea tree oil and the copper & vinegar are both worth consideration. In both cases, they work by disrupting the fungus’ ability to reproduce/replace lost cells, which means the fungus withers away because it can’t sustain itself.
The catch: for tea tree oil, you have to apply it every day for 12-18 months, even if the symptoms disappear before that time. For copper oxide, it may stain your skin and hair green. Personally, I’d go for tea tree oil. It costs more, but there have been plenty of reliable studies done on it to prove that it works against any kind of fungus. If the rot is really bad, you could also supplement every day with a few drops of oregano oil taken orally. Then you hit the problem from both sides of the skin, so to speak.