I've been through the same as well, i also tried Macrogol which is the same as lactulose just more effective and is being sold in sachets instead of liquid.
I also tried 2 bottles of duphalac in the past. Macrogol works a little better but stools are painful for some reason i cant understand.

Try using parafin mineral oil, i had a huge difference using it this summer, when i consumed around a litter of this oil in less than a month, when the fissure was still very painful, it did help.

Basically what it does is lubricating the stools and making it softer. You end up with a huge pile of stools underneath and you do not realize when they did come out of your butt.

Although it works best and can be used long term, i guess that you have to cycle it with duphalac or macrogol (i use a local cheap brand called Effecol in Greece but i think movicol or miralax are the same).

Last week i've been through a case of diarrhea for a couple of days and it followed 3-4 hard BMs. I used macrogol but it wouldn't help and i almost ripped it again, even using enemas and glycerin suppositories wouldn't help. Then i remembered the mineral oil and finally it helped soften the stools.

I have the fissure around 8 months as well, starting back in June 2015.
I guess we have a lot in common
Good luck