Maybe you should try Bactrim instead. Just go to your doctor one more time and tell him you think your problem is prostatitis. If he agrees, then he should know it will take an extended period of 30 to 45 days on Bactrim DS, double strength to rid your prostate of the infection.
The best thing about using your doctor one more time is you can get your prescription filled at Wal Mart and get a generic Bactrim, SMZ/TMP DS 800-106 antibiotic for like $4.00 for a months supply.
Once the doctor sees that you suspect prostatitis and that explains why your UTI keeps coming back, and he should prescribe the extended treatment plan. Like I said, I would try Bactrim DS.
I actually ended up using the Canadian online source, because even though they were expensive, it still beat the cost of a doctors appointment plus the drugs.