Michaeleen Doucleff For decades, psychologists believed willpower was the ticket to a good life. "It was thought that people with better willpower would be more successful," says psychologist Marina Milyavskaya at Carleton University, in Ottawa, Canada. Hundreds of studies appeared to support this idea. Researchers found links between better willpower and better grades in school, better relationships and careers as adults, healthier diets and even more consistent parenting. So psychologists and parenting experts advised parents to teach children to use willpower to resist modern temptations, such as sweets, fast food, video games, phones and other screens. But in the past 15 years, Milyavskaya and other psychologists have dug deeper into the studies, and they uncovered a major flaw: These studies weren't actually measuring willpower but a different skill — the ability to avoid temptation in the first place.…