You hang in there. There are some things I learned in this process that may help you to keep in mind.
First, it can be very easy to re-tear early on after LIS. But it is also easier to heal after re-tears as well.
While the initial healing of the fissure is quick (within 4-5 days), full healing take up to two years. So, you can't really "go back to normal" after a few days or weeks. You need to stick with your softening regimen (only modified as needed) as you did prior to LIS for at least the next 8-12 months.
As your doctor said, having had this procedure will require life-long changes in your bowel habits. Some of the things I did and still do (and will be doing for life) include:
1. Lots of water - I get in at least 100oz daily.
2. Start everymorning with Benefiber in my yogurt.
3. Nightly dose of Miralax (currently 1/2 to 1 full dose). I will forever have to do this because I take a lot of calcium daily which is constipating (caused my first fissure). I'll be taking the calcium for life, so I have to do what's necessary to protect myself from the effects.
4. Invest in moist wipes (I used Pampers Sensitive). I use this instead of toilet paper in the back.
5. For the first year, I did 2-3 warm soaks daily. Now I still do a nice, long hot soak after my morning movement. It's become a ritual.
Beyond these things, I am mindful of any differnce in feeling back there. If I feel the slightest stinging, tenderness, burning, etc., I'm right on it with A&D ointment or something similar. Also, I keep a couple bottles of Witch Hazel in the house always and add some to my bath water for my soak. I will always be taking these steps as I never, ever want to have a nother fissure as long as I live.
Don't get discouraged, just realize that healing takes time and has ups and downs and you'll be fine.