I'm sorry to hear you had such a negative experience with that treatment
For me it changed everything. I had botox injected only once into my urethral sphincters 12 years ago, because they had spasmic contractions and would close aggressively at the least urge. This was caused by a TURP (prostate resection) and one of the nerves got hit.
It took a week or 2 before the medicine worked properly, which is also what the doctor verified, but for me it worked about a year and a half (well, not really, botox only works for 6 months but the sphincter was affected long enough to adapt).
After that I also had done a plethora of sphincter relaxation phisiotherapy and I no longer have such severe case of spasms.
I don't think repeated botox would benefit much more though, and if it isn't working for you I'd definitly look into other areas.
It also depends on the skill of the surgeon, I had 8 urethrotomies and 2 of those included a TURP, of those only 1 had a very low scar tissue regrowth, (different hospital, different surgeon).
In my case having it injected into the bladder itself would be fatal, I already had severe atonic bladder after one of the surgeries and they removed the catheter, I could not void anymore and the next day had to rush off to ER for drainage :s