Hi JJoo I am so sorry to hear about your problems and know how painful cystitis can be.
Have you actually had a urine test to see which antibiotic is best for your specific bacteria? When I last had a problem, I was prescribed a routine antibiotic which did nothing, then a 2nd course of the same one, which also did nothing.
Eventually I saw a different GP who sent off a sample, and it turned out that the cystitis was being caused by E.Coli, and I needed a different antibiotic to specifically deal with E.Coli. I had been suffering for weeks before a doctor did a proper check. I know you have been suffering for much longer.
Now I keep D.Mannose tablets in the house and at the slightest twinge I take some.
Cranberry juice was always supposed to help but I believe that theory has since been disproven. It is also quite acidic which possibly doesn't help. You need to make your urine alkaline, so Bicarbonate of Soda mixed in water and drunk several times a day will help, and drink as much water as you can manage.
I hope you can get something sorted out. for you.