Hi!
Gosh, you do have a decision to make based on the experience you have had.
On the one hand, I can understand totally why you would prefer to forget the surgical intervention and leave the fistula alone and leave things as they are because you are pain free and feel normal, but on the otherhand, a fistula will not go away without the surgical intervention and over time, there is also the possibility that it can grow or develop further tracts and you don't want the risk of that happening either, which in turn, could cause you further complications.
I would suggest maybe go back with all your worries and fears given your history and get some thorough answers and reassurance to your queries.
I had a fissure, which was healing when my fistula appeared and was using Dilitazem at the time to treat the fissure.
I still opted for the Fistulomy because I just didn't want it there. It also didn't cause me much pain and I was suffering only with the discharge stuff but nevertheless, I wanted it sorted (but that's me!) Also my fistula was totally unrelated to the fissure and in a completely different area, so I didn't have to worry about that connection like you.
The procedure for a Fistulomy is very different to treating a Fissure but also highly effective and successful in removing it when treated properly. It could be that you have nothing to fear and surgery will deal with it without causing further problems with the fissure. I believe that sometimes the creams are prescribed to people dealing with Fistula recovery but still suffering the ails of a fissure to relieve the muscle spasms in the anal area and aid the general healing process, but you really would need to go back and ask the questions to get that confirmation and go from there.
Truly all the best to you in whatever you decision you make but get some more medical reassurance on those existing worries that you have. Hoping it all works out for you and you make that decision soon. :)