I am sorry to hear you are worried about Lucy. I am a veterinarian and will do my best to help.
Most yeast infections are caused by an underlying problem such as allergies, or autoimmune skin disease. If you do not get the underlying problem under control, you just keep seeing these yeast infections coming back.
There is a new medication, called Apoquel, that is really great for allergies, and helps about 80% of allergic dogs. If they stop scratching due to allergy, then your yeast treatments will work, but the problem is less likely to come back. I would ask your vet about trying it.
There are supplements that may help: nettles, quercetin, fish oil, anti oxidants, and probiotics. Combined with using yeast medication, and something to control the underlying allergy/itch (Apoquel, prednisone, antihistamines, cyclosporine) may help get this controlled. Oral fluconazole is another thing to try. You could try the Monistat, that would be safe, but alone it will not cure her.
Let me know what else I can answer or help with.