Agree with @yerrag, Magnesium is often key with hard stools, especially when it's the chronic dark, hard and dry, even though i don't that is the OP's problem in this case.
Full doses of milk of magnesia or "USP" grade epsom salt as per package instructions. Or if you don't want that sudden effect you can microdose these several times per day with 1/4 or 1/5th doses. Other Magnesiums would work but their labels won't contain instructions on how to deal with constipation ... you'd have to look that up yourself or experiment.