Prenatal vitamins and constipation

Laurie
I'm currently 29 + 2 and struggling with constipation, which lead to some ugly and miserable hemorrhoids.  My diet is healthy and has lots of fruit, veggies, wheat bread, activia, etc.  I also drink plenty of water each day.  I've tried colace, prune juice, and trying to eat a lot of fiber, but nothing softens my pebble like, rock hard bowel movements.  The only thing I haven't tried is switching my prenatal. I'm currently using 'one a day' prenatal.
ANY ADVICE ON ANY OF THIS IS MUCH APPRECIATED:). Don't know if I can do this for 2.5 more months:(