Daughter is constipated?

I have a 3 year old who we've been trying to potty train for a little while now. She's only got down the bowel movements so far. But lately she's been very constipated and refuses to push on the potty. We have to sit there for hours on end until she has the urge to go, but then its been painful for her in the past so she squeezes it back in and refuses to push. Any ideas on how I can get her to go without seeming like I'm being the mean mommy? Two days ago I gave her a Glycerin sipository because it was a week and she hadn't used the bathroom, but I really don't want to give it to her everyday and make her become dependant on them. The pharmasist gave us some medicine that pulls more water into the bowels and it seems to work whenever I give it to her, but if I dont, she's back to being constipated again. I really dont want to have a child dependant on laxatives her whole life, and we really need to start getting her going consistantly on the potty.