Another vote for Fibresure or fibre 89. They are different things (one psyllium husk, the other ...not) so I alternate, seems to stop system getting bored or used to one thing. Just one tsp of F89 in morning shake, or if not, 2 Fibresure capsules with morning shake.
If it hurts and you neeeeeeeed to go, then sort that out first before adding the fibre additives to your shakes, as that will only compound the problem.