Maurice:
It is good to know that one is not alone with these problems -- this forum is great for that.
My best guess from hearing about the large amount of pain that you experienced with the developing fistula, is that the post-op pain will likely not be near that.
I don't mean to paint a rosy picture of the post-op period, which comes with its own pain and aches, but for me it was definitely better than that pain I remembered when my fistula developed and the sharp and anxiety-driving pain of the fissure along with the other complications that came with it the past couple of years.
Plus, if you couldn't sleep due to the pain of getting the fistula and it was making you ill -- my post-op period has been the opposite of that. I am not back to my full active self yet but...I slept tons the first week (also thanks to the pain meds), the pain lessened each day, and I am continuing to pamper myself (enjoying doing nothing all day) and taking it easy on my bum so that it gets a chance to heal all the way.
My post-op pain is nowhere near the pain of when my rectal problems got bad. Best of all, it is manageable and predictable, unlike worsening rectal problems that may not go away, come on their own terms, and flare up when you get stressed.
Taking care of this now rather than later sounds like a good idea -- I waited 19 years to do the fistulotomy and I should have done it earlier. Even when it was not painful for many years afterwards, it made it harder to wipe and irritated the tissue down there more.
Best of luck and let us know how the procedure goes!