It sounds like prostatitis, but hopefully that should be impossible.
Has anyone cultured the UTI and identified what it is, so they can select an antibiotic it's sensitive to?
If it comes back once a month, I suspect it never went, and if it is a bacterial infection, it wasn't sensitive to the antibiotic used. Many antibiotics are also anti-inflammatory, and some temporary relief might have been derived through the anti-inflammatory properties, rather than the antibiotic properties.
Something else that can generate repeated UTIs is incomplete voiding - not emptying your bladder when peeing. This is because the residual acts as a reservoir to keep any infection going. Also, drinking too little fluid so you don't have a good flow through urinary system allows more bacteria to build up.
Drinking cranberry juice can help avoid UTIs because it coats the bladder and urethra with a chemical which prevents bacteria from sticking, so they're more likely to be washed out when you pee. (However, that won't work if you are not emptying your bladder.) Go for a sugar-free one (otherwise the high sugar may defeat the object), and beware if any other meds rule out use of cranberries (such as Warfarin).