Hi Ginny42, I have also suffered frequent cystitis & kidney infections (from age 6!), and I have no magic solutions, but I think it's worth you looking at NHS Choices on cystitis, and you might also want to click on the link for "interstitial cystitis". I've had 4 hospital-based investigations (instigated by GP, not me) over 5 decades, and I find it very frustrating because I think they just want to cover their backs in case you have a tumour or something sinister. When they don't find anything sinister, their attitude is 'job done' and you are left to manage as best you can. Here's my routine when symptoms strike: paracetamol (not ibuprofen - can damage kidneys and gut); hot packs - one behind the small of the back and one over the kidneys; loads of water to drink; Cystopurin to alkalise the urine and stop the stinging; here's one you might find strange, but work for me - Germaloids haemorrhoid ointment applied to the vulva to stop stinging when you pee.
Cranberry drinks have been found to be no better than placebo in large scale trials, so do it if you want to. I think doctors still feel it's worth telling you this because it might make you drink more fluids and it gives you something to do.
One thing I have to disagree with is the idea that cutting yeast out of the diet works. It is based on the mistaken idea that any yeast can cause infection. Well, it can't, no more than you can catch dry-rot from handling floor-boards! Baker's yeast is Saccharomyces cerevisiae and the organism which causes "Thrush" is Candida Albans. They are both yeasts, but are about as closely related as cabbages and oak trees! Please don't deny yourself on the grounds of a fallacy. I can recommend reading 'Bad Science' by Ben Goldacre for a superb insight into persistent quackery. When you have a chronic relapsing condition, it is all too easy to think that some quackery or another 'worked' for you, but it's only the normal remission of the disease (until the next time).