fistula is a disease that needs surgery...
but as u said u want to avoid it then only medicines that will work will be stool softners and hot sitz bath( sitting in warm water)
these will only stop further infection and avoid another fistula to develop.
Next Steps
take high fibre diet and avoid constipation
get an mri fistulogram done
Health Tips
Nowadays we do laser fistula surgery that is less painful
so u can go ahead with surgery because permanent solution is surgery only not medicines