When I complained to my GP about thrush in the breast I asked for the fluconzaline as suggested in the Breast feeding network link mentioned above.
This is the same as the Canestan tablets that you can buy over the counter.
The single dose didn't work for me but I ordered the generic version (at least £8 cheaper than the Over the counter Canestan) and followed the protocol from the breastfeeding link to keep medicating each day.
This in combination with using the grapefruit seed extract really seemed to help - my symptoms cleared up within 4-5 days. It sounds like your grapefruit seed extract protocol you're following is already good.
You seem to be doing everything right so I'd suggest trying to clear from the inside with the Fluconzaline (spelt wrong!)
You have my sympathies.. this happen to me when my daughter was around 1.5 and the pain was agony. Luckily for me she is well established on solids so I could say no to nursing... one breast in particular was really sore for me so I limited feeds on that side.
I had no issues with supply or blocked ducts but I attribute that to nursing a toddler - if your baby is younger you're going to have to be more careful. I really do feel for you because it was agony and I had no ideas how mamas of younger babies continued nursing through the pain!
Good luck and keep on keeping on... you've come so far that it's worth one more try to get rid of your symptoms.