That is a loaded question. A fissure can take 6-8 weeks for initial healing but can take up to a year for complete healing to occur. That doesn't mean that the agony will continue that long but that you have to act like you have a fissure longer than the symptoms last. Plan on doing these things long term in terms of the diet and baths etc.
Maybe they can try giving you oral valium as a muscle relaxant since your bottom doesn't like the creams. I have had a few doctors give me very low doses to help with spasms.

wow I've had that stuff before when I had an operation go wrong in hospital, it would certainly take my mind off all the pain! Only joking, I suppose at low doses it wouldn't space you out so much, or does it? Can you get reliant on it?

I was given something like 10 mg to take after going to the bathroom which is a really low dose. I don't remember feeling spacey or buzzed at all. It is considered an addictive drug so many doctors are reluctant to give it out but at such a low dose for a short period of time I don't think you would get addicted/dependent.

A note on the nitro: from what I've read, it's supposed to sting for the first few minutes. That's how you know it is working. It stung me too when I applied it, and the stinging lasted anywhere between 30 sec and 30 min.

It's job is to draw blood into the area, which could account for the redness you saw.

If you are going to try it again, maybe you could cut it further with some vaseline. 1 part 2% nitro to 1 part petroleum jelly=1% nitro. Perhaps a lower doseage would work better for you.