I am so sorry that you are suffering so much from UTI's. As common as they are in women, most of us don't get them every 2 weeks. I feel your pain, I have had UTI's in the past and they are by no means a walk in the park.
The problem with a woman's urethra is that it is VERY short. This means that bacteria are prone to entering our system at any given moment. I read the precautions you are taking including your medications, but I also wanted to point out some other ways you can help prevent a UTI:
-No tight pants or jeans
-Go to the bathroom as soon as you feel the urge to urinate
-Wipe from front to back-- ALWAYS
-Avoid feminine hygiene products (for example: summer's eve) Contrary to popular belief-- these hygiene products are far worse than we think they are. They interfere with the normal flora of our bodies and thereby create bacteria that was not there to begin with.
-Empty your bladder immediately after sex-- bacteria from having sex should not be allowed to linger
-Continue to do the other things you've already been doing-- Lots of water, white cotton panties (allow for breathing room). Don't OVERwash. Many women think that by obssesively washing they will fix the problem when in fact it may make it worse.