Psyllium husk is a fiber adder and can bulk up stool. By itself I'm not convinced it'll help with a fissure. It could make things worse if your stool only becomes bigger and not softer.
I have to admit that I don't understand why the doc would tell you not to use a stool softener (laxative). A good stool softener is critical for fissures.
As for the age thing, I'm a bit confused there too. I was 31 when I had my fissure and no doc ever expressed any concern over surgery. I eventually had it done and it worked out nicely. Lots of other young folks have had it too.
A lot of us were given suboptimal advice early on before figuring out what really worked for our fissures. Ultimately it's your body and your call, of course, but if what the doc told you to do isn't working, then at some point you have to question why, right?