Senna should only be used for a max of 2 weeks as it can cause lazy bowel. Senna is a stimulat, so if the stool is hard it is going to be painful coming out. You would do better with a softener like sodium docusate or one that works by osmosis like Movicol. Only problem with Sodium docusate is it works by mixing the dietry fat with the poo and as we don't have a lot of dietry fat it might not work so well, whereas Movicol pulls water into the bowel and keeps the poo nice and soft.
Hope this helps a little.
Charlie xx