Hey, Im prone to get cystitus too, so I know how you feel. During year 13, I was plagued by it.
I found that cranberry juice was a god send. Takes a while to learn to like it though.
Tea, probably not the best thing, as it messes around with your antidiuretic hormone (ADH) and, can make your cystitus worse. I stayed far far away from tea when i was getting it lots. If its the relief of a warm drink, try a warm juice or something like that maybe.
I have somethign from the doctor that says
1) take a pint of water when you notice the onset of symptoms
2) every 20 minutes take at least another half a pint - I know that this is a lot - and it should releave the symptoms.
3) wrap a hot water bottle in a towel and place on the lower back or between the thighs to relieve pain
4) Take a teaspoon of Bicarbonate of sofa with half a pint of water every hour to make your urine less acidic and help to stop breeding bacteria.
This leaflet also says if you're prone to it to:
1) drink AT LEAST 3 pints of water a day
2) pass water whenever you need to, dont hold onto it
3) make sure you empty your bladder completely
4) always wipe from front to back, to avoid spreading bacteria from the anus to the genital area
5) avoid perfumed soaps/deodourants/talc in that area (I think its best to not use anything at all but water, though there are some special ones for that area, but they can alter your ph balance too.
If you get cystitus after sex (which you do) it may help to:
Wash and pass water BEFORE and AFTER sex and ask your partner to do the same (I know its not really romantic saying "can you wash first please" but it really could help, and you could possibly make it sexy? )
Use a lubricant during penetrative sex.
That's the NHS leaflet that I have. I know it sounds silly, but it could be anything to a little bit of dirt on his hands or anything. So maybe you could try and incorprotate the bathroom into your sex life?
Definitely go to the doctors, they dont always piece together that its your 6th time there for the same thing until you actually say to them, that youve been there loads before. There are things that can be done.
Hope this sorts out, I did for me, and touch wood Ive not had a serious attack in a few years now.