Hi, I suffer with recurrent cystitis like you do and have done for the past ten years, since I was 17. I don't think I changed the way I lived but anyway, the cystitis kept recurring. I saw specialists but to no avail, anyway last year I went to a doctor who is also a homeopath. He put me on a program to kill candida where i cleansed my bowels through with medicine, supported my liver, killed the candida and took probiotics to build up the good bacteria in my body. I also discovered that I was intolerant to sugar - which plays the biggest part in me getting cystitis. I avoid sugar - it sounds drastic, but to me I cannot cope with getting anymore cystitis, I was/am desperate to not get it anymore. I would recommend you see a doctor who has also trained as a homepath then you know you're getting the best of both worlds. If you need a sugar hit then xylitol is safe to eat. I also have an underactive thyroid but started getting cystitis before this became noticeable. A good test for me was the antibodies peroxidaise. If you have any level of TSH in your system then it shows things aren't working right - just something to look out for.
Also, i came off the pill - progesterone - found in all contraceptive pills will feed the bad bacteria (the candida). Candida will suppress the immune system and make it struggle.
Best wishes
xxx
ps; there is no evidence but it is thought that antibiotics, whilst being an amazing wonder drug, are also responsible for some of the damage to our immune system because they kill everything. If you ever have antibiotics again, make sure you take probiotics during and afterwards for 2/3 weeks. If you're going to kill all bad and good bacteria in your body by taking antibiotics, then you need to make sure you replenish the good by taking probiotics - or the bad bacteria (candida) will just grow again. I hope I make sense. This is not new science - go and visit your local health store - they will know all about this.