Hi mibbes & fellow sufferers.
Really sorry to hear you are still in agony after 6+ weeks. I can relate to this as I had the same experience (though not necessarily exactly the same symptoms) with DS. I remember going to the GP for my 6-week check desperately hoping that it would be thrush - I was gutted when he ruled it out but I made him take a swab anyway. It turned out not to be thrush but the problem is that you can never tell which HCPs are clued up about bf stuff and which aren't.
The problem in our case turned out to be tongue tie. You really need someone who knows their stuff to have a look at this to rule it out. With DS (and DD - also tongue tied) it was missed by the hospital paediatricians, the GP and too many midwives to count before it was finally diagnosed. Perhaps you could post on your Mumsnet local page to see if anyone can make a recommendation of a HCP who is experienced in diagnosing tongue tie. I would certainly not trust just any old midwife or GP, or even someone running a bf drop-in.
It's not easy to diagnose, but when I took DD to have hers snipped the other week the consultant showed me one of the things he used to diagnose it. He waited until DD cried and showed me a little dip in the end of the tongue where its movement was restricted. It was so subtle though in DD's case. By 6 weeks DS had a notch on the end of his tongue (still has) where the tie had restricted its growth. Not saying that if your DS doesn't have any of these it is not tongue tie - it is worth getting it checked out properly anyway, even if you have been told previously there is no tongue tie.
Whatever the underlying problem though, improvements in latch and positioning can make the pain more bearable. If you're getting more pain on one side then it's worth having a close look at the angle of the baby against your body. In my case I noticed that on the less painful side I was holding DD almost at a 45 degree angle to me (ie with bottom tucked right in almost under the armpit) and on the other side I was holding her pretty much straight across me. Changing that made a big difference. Another top tip given to me by an infant feeding specialist is instead of arranging cushions then putting the baby on, get the baby on first then slot in cushions afterwards to support your arms & baby. I was surprised how much difference this small thing made, but it's all to do with the old baby to breast, not breast to baby thing. If you do breast to baby, then you tend to end up slightly hunched over but then once the baby is feeding, you lean back ever so slightly without even realising it, which pulls the nipple away from the baby. For the same reason, it's worth seeing if resting your feet on something helps at all. I've never used a feeding pillow, but I would think the same applies - perhaps using smaller cushions/rolled up blankets/whatever placed strategically might work better than one big pillow.
Other than that, painkillers helped me during those dark times (alternating ibuprofen & paracetamol). I also used nipple shields for 3 weeks with DS - they have their own drawbacks but it was either that or give up at the time. Neither will solve the underlying problem of course, but they might buy you a little time while you're looking for the solution.
I think you're all doing brilliantly for persevering - a lot of people would have given up long ago. I really hope you find the answers soon.