Hey, I'm on Ditropan XL and it has improved my frequency greatly, with no side effects at all. I never had any problems with urinary retention, though.
What also worked very well for me was bladder training. My urologist did not even suggest this, they just love to prescribe pills. I found it and a lot of other great information on the Interstitial Association website. Your bladder muscle is like any other muscle. It needs to be worked in order to retain its tone. If you pee every time you feel the urge, your bladder loses tone and it actually makes things worse.
I was peeing numerous times an hour, just like you, I was miserable. Then I put myself on a schedule and recorded when I peed. At first, my goal was to 'survive' for 1/2 hour in between pees. I peed at 12pm, 12:30pm etc. Eventually I was able to go for 1 hour in between pees. Eventually I worked it up to 2 hours. Within 1 week of doing this, my daytime frequency had improved greatly and I was much much happier. I still have to take the Ditropan XL for the nighttime frequency. You can try bladder training in addition to taking medication.
Other things that have helped me: remove all caffeine from my diet, limit sugar, limit citrus fruits, limit spicy foods. This is all ways of treating interstitial cystitis, but also help with overactive bladder. Unless you have a cytoscopy under general anesthetic, you will not get a definite diagnosis of interstitial cystitis.
BTW, my symptoms used to be much, much worse just before my period and then improve when I got my period. No idea why though.