You mentioned having a lot of medical problems with her–was she treated with antibiotics b4 the yeast appeared? Antibiotics can kill beneficial bacteria that keep yeast under control in the intestines, the same as with humans. If treating her externally hasn't helped, she might have a systemic (internal/whole body) yeast infection. Have you tried giving her plain probiotic yogurt--the kind in the grocery store they advertise as having "friendly" bacteria?
Also, being young, your girl's probably had a lot of emotional & systemicl stress in the last year, moving to a new home, trips to the vet or groomer, vaccines, heartworm chewables, maybe flea treatments...maybe she was exposed to pesticides/disinfectants at the puppymill she came from. All of these can affect a young dog's immune system, which can cause yeast overgrowth. A few years in a healthy environment with healthy food will do wonders for her immune system.
I have a female B that was bred in a puppy mill and while the first two years were rough (food allergies, immune problems) she's now 14 and the love of my life. (Don't tell my husband that!)
Here's a great website that deals with yeast issues in great danes...keep in mind that there are healthy commercial diets and raw meat diets that aren't so hard on the wallet, too.
Good luck & hang in there!