Baumeister and Tierney start off Willpower with the comment that
"We have no way of knowing how much our ancestors exercised self-control in the days before beepers and experimental psychologists, but it seems likely that they were under less strain. During the Middle Ages, most people were peasants who put in long, dull days in the field, frequently accompanied by prodigious amounts of ale. They weren't angling for promotions at work or trying to climb the social ladder, so there wasn't a premium on diligence (or a great need for sobriety). Their villages didn't offer many obvious temptations beyond alcohol, sex, or plain old sloth. Virtue was generally enforced by a desire to avoid public disgrace rather than by any zeal to achieve human perfection. In the medieval Catholic Church, salvation depended more on being part of the group and keeping up with the standard rituals rather than on heroic acts of willpower." (p.4)
While I am not a scholar of the Middle Ages, this struck me as somewhat simplistic. A key feature of the Middle Ages was war - in the Holy Land, among the Italian city-states, against the King of France by his vassals, among different claimants for the throne of England. At any point in time, men could be called up to fight for their liege lord, or endure armies trampling their crops and looting their houses.
Farming itself is stressful. A drought, too much rain, or a freak hail storm could quickly destroy a crop, create hunger over the winter, and lead to deaths from starvation or disease brought on by malnutrition. It is also physically taxing in a time when there were few ways to relieve pain from arthritis or poorly-set broken limbs. A simple cut could fester and cause death. A cold could bring on pneumonia. Childbirth fever killed many women.
What may have made a difference in the Middle Ages was diet. Everyone ate unprocessed, organically grown, local food. Peasants could afford few sweeteners (such as honey) or sweet fruits, and subsisted almost entirely on whole grains. If they did eat meat, it was from a free-range, organically fed animal. They may have had little food, but it was healthy. The lack of sugar, the quality of the carbohydrates, and the protein would have led to fewer glucose spikes. This would have led to less depletion of willpower due to inadequent glucose. Diet, not lack of stress in daily life, could have given our medieval ancestors more willpower.
There were few material goods in the Middle Ages. The average person wore home-made clothes, ate home-grown food, and rarely set foot in a shop or even browsed at a marketstall. Members of religious orders were sworn to poverty. The emphasis in society was not on consumption, in part because of religious teachings and in part because resources were so limited. A person who hoarded rocks might find that family members confiscated the rocks to mend walls in pastures. A woman who hoarded cats would be prosecuted as a witch (along with the cats).