Fissures can be a real pain to heal (no pun intended...well sorta :-)
Preventing hard BM's is really the best way to heal them over time. Do your best to relax while passing. It's really tough to relax when you know the pain is coming. Clinching and bearing down is only going to make it worse.
I had 2 at once (12 o'clock and 6 o'clock). I had them for probably 6-7 months. It's a tough road. Hang in there.