Hi. I had this problem with both of my children. Here are my comments/suggestions.
1. Yeast is highly contagious! You have to be dilligent about hand washing Anything that touches your breast or baby's mouth needs to be washed in hot water and dried completely. You can use white vinegar to wipe down any surfaces/toys that come in contact with the yeast. I used to wipe down door handles, knobs of cabinets, and my counter tops with white vinegar (it kills yeast on surfaces).
2. Call a La Leche League mom. Your doctor's office will have their number. You can also look them up on the web.
3. Dr. Sears has a website as does Dr. Weil. They both give good information about yeast.
4. Go back to your doctor. Explain everything you have gone through. As for an agressive treatment and anything else that will help. I ended up having to scrape the thrush off the tongue of my infant after every feeding. I used a tongue depressor that the doctor provided and scraped as much off as I could. I'd toss it, wash my hands and administer the Nystatin. Then I'd wash up again and apply the Nystatin cream to my breasts.
5. Go to the health food store. Get acedopholis (mispelled, sorry), a probiotic. It puts the good bacteria back into your system to help fight the thrush. This is for you only. I don't know that it's safe for babies.
6. Cut out sugar. Eat fresh veggies and fruits. Drink a lot of water. No dairy except for yogurt with live and active cultures. I was on a high sugar/carb diet but changed because of the yeast...it's worth the sacrifice and I even lost a couple of pounds!
7. Keep your breasts dry. Use disposable breat pads after your nipples are dry from a feeding. Change the pads after each feeding/application of Nystatin.
I know that's a lot but yeast is a tough nut to crack. You can do this.