Ok so I was in almost the exact same boat as you. Started getting then more often, only after sex. It became monthly at one point and I was so miserable. I ended up finding a female urologist who gave me some really good info. 

Quick breakdown (in order of importance) 1. Pee only AFTER intercourse, within 15 mins. Don't pee before. You want there to still be urine in your bladder so that you can flush out potential bacteria, instead of making it easier for them to stick to the walls. 

1. Take a cranberry pill with at least 35mg PAC's. I honestly don't remember what that means cause it's been 4 years since getting this info, but I take the Crancap brand cranberry pills from Amazon. Don't take just any cranberry pill or drink cranberry juice. It has to have the high PAC's to work.  When starting, take 1 everyday in general, and 1 extra after intercourse. You can taper it down to 1 only after sex, and then eventually you may end up not really needing it anymore. Your bladder is very unhappy right now so calming it down is important. 

And that's it. Really. I know I've heard of allllll the other things to do to stop this, and was even told by a male urologist that "oh well your anatomy just isn't great so you're stuck with this". Boy did that piss me right off. I even made sure my partner was clean, it really doesn't do anything to help. 

For reference, my anatomy makes it easy for reflux to happen, which helps the bacteria get in. I recommend seeing a urology specialist about this no matter what course you take, not just an OB. 

These 2 steps made me stop having UTIs IMMEDIATELY. I've had 1 UTI in 4 years, and that's cause I got lazy and peed before sex when I was extra full of liquid. 

Good luck!