I've had a fissure for several years now, which the colorectal doctor hoped would heal but finally gave up on. The only thing he ever gave me that helped was something called "topical diltiazem." It's a salve that serves as a muscle relaxant to prevent your rectal muscles from going into spasm when you use the bathroom. It isn't a pain reliever, but it did seem to make the whole area a little calmer and less painful after a few days of use.
I haven't had any for a couple of years, but I've been thinking about asking for a prescription again (since I occasionally feel as if I'm passing a razorblade). I had to have it specially made up at the only pharmacy in town that will create special-order concoctions by doctor's request. It may be worth your while to ask your doctor to prescribe it for you.