Sorry to hear of your setback but my CRS did tell me that even after the LIS you will still occasionally have a fissure and it's fairly normal. Remember your fissure didn't magically close because of the surgery. The surgery is simply helping the body heal the fissure (as it does for anyone with an acute fissure). I honestly think you'll be fine. Stay the course and again you do not want to be dependent on laxatives. You will mess up your colon!
Try this mental approach. Pretend this current episode is a brand new fissure. Tell yourself it will not be chronic. Do your few sitz baths a day and do not do anything extreme. I am serious - it's what I would do. You had an LIS, your body will heal itself quickly but if you start freaking out and getting tense down there you are screwed. Again, treat it as an acute new fissure and it should be ok by tomorrow.