I have suffered on and off from cystitis for years. I will type out a few points and if I go over old ground then I apologise.
You must drink lots of water not just in the lead up to sex but all day every day. This is to ensure that anything that irritates you is flushed away.
Wipe everytime you pee, unscented toilet paper is a good idea and wipe front to back never back to front.
Try not to use scented shower gels or bubble bath. Try not to clean too much internally, it will only damage the lining, remove your natural mucus leaving you dry and cause further irritation.
If you are using condoms, you dont say so I dont know, try latex free ones and ones that dont contain a spermicidal agent. These can also cause irritation especially if you are allergic to latex.
You must pee before having sex and afterwards never go to sleep before you have. Also drink plenty of water after having sex so you either go first thing in the morning or during the night.
Use lube not just a bit but gallons of the stuff. Putting it on at the beginning in the hope it will last throughout your bedroom gymnastics is a no no. You have to apply regularly and liberally, any sense of dryness or discomfort, stop and apply more.
You may have to try different sexual positions. After discussions with my OH, we found he was too high up during missionary and just bending his arms slightly at the elbow made a huge difference. Try doggy, riding him and other positions see what works best for you.
Also if your OH is fingering you his fingers also have to be clean, as in nail brush scubbed clean. I love to be fingered but even that had to be modified so it wasnt so hard and fast and rough as I like (sorry to be so graghic) as that also brought on an attack.
I would also suggest you have cystitis powders in the house and your handbag so you can take action promptly.
I hope some of the suggestions work for you and you dont have to resort to antibiotics all the time and my apologies if I have repeated suggestions posted earlier.
LMF xx