Lottie had cystitis 6/7 times, it was so bad she quickly went to passing clots of blood and the last couple of lots lasted a month. Each time the vet gave her some cystease for 2/3 weeks and ABs. After it had occurred three times vet suggested taking a urine sample and xray to rule out crystals/stones/infection/cancer. They came back clear, so at that point we could be sure it was stress related/unexplained cystitis. I bought Feliway diffusers which made no difference to her but for some it really helps. In the end I bought some Cystease online and put her constantly on them, it lines the bladder and reduces irritation. I have to admit I've never taken her off a maintenance dose (approx five a week), the vet knows this and doesn't have a problem with it.
If it keeps reoccurring then, I'd say you do have to exclude other conditions (as cystease alone won't be enough for some). Also, try Feliway if it's stress related, it might help. It's not easy going through this, but somehow we got through the other end and Lottie has gone six years without a re-occurence - at 18 she does have other health problems but nothing that relates to the bladder or cystease.