I had terrible problems with a deep breast thrush infection following the use of antibiotics to clear up a bout of mastitis in week 3. The infection lasted until week 16, and made feeding excruciating - like burning hot oil running through both breasts, before, during and after feeding. I do so sympathise!!!
In my case, I don't think it was a diet thing, although interestingly I do eat a lot of the 'bad' things at the top of the lists shown below. Mine was also not due to poor positioning - something which you do need to get checked out, if only to ensure that health professionals take your cries for help seriously.
My GP was very unsympathetic - basically disbelieving that deep breast thrush infections were possible. IHO they could only be surface infections. There wasn't any point in my taking something orally, as not enough of it would reach my milk ducts, apparently.
From week 3 - week 15, I went through various nystatin courses (cream for me, oral suspension for dd) none of which worked. Despite ample literature provided by a fantastic NCT b/f counsellor and the BFN, my GP wouldn't proscribe anything else. It seems that flucanazole has been tested on women, and infants, but not on b/f mums, so he was reluctant to proscribe. Also, most of the studies are American. Eventually, though, I got an appt with a locum who was rather more rational, and whom I managed to convince to let me try something else. He calculated that if the dose was OK for infants, it had to be ok for babies being breastfed, as the amount they'd get would be so small. He proscribed me flucanazole.
The crazy thing was that all it took was 1 tablet of Diflucan (if I'd known it was commercially available, I would have bought it myself I was so desperate!!). It cleared up within 2 days.
I appreciate that GPs have to be careful about what they prescribe, and mine was extremely apologetic after the event. The good thing was that my HVs used the case to 'educate' the GPS about the incidence of deep breast thrush infections. They said that the problem is that most mums in that much pain just give up b/f, so GPS don't tend to see many people needing treatment. Hence there aren't enough people to test out particular courses of action and draw conclusions.
I don't know if my experience helps, other than to underline that there are others who've been through it. I went on to b/f dd until she was 10 months old, and decided to give up herself, and I'm so proud I stuck it out. I'm not sure I could have gone on much longer though, and I can see why many people would have given up sooner. I think I'm just a bit bloody minded!!
I hope you get the support you need.