The researchers show that people prompted to write about how a positive event may not have happened experience a greater uptick in mood than those prompted to describe the positive event. In their most persuasive study, individuals in committed relationships wrote for 15 to 20 minutes about how they might never have met and connected with their partners. Others wrote instead about the reverse – that is, how they did meet, start dating, and end up with their partners. Several control conditions, which involved writing about one’s typical day or about one’s friendships, were included as well. The biggest increase in satisfaction with the relationship occurred not in the group that pondered the sunny beginnings of their union but in the “mental subtraction” (or “How I might never have met Peter”) group.I suspect this works in reverse as well ("...if only I hadn't bought that car..." or "...if only we had just stayed friends instead of dating...").
I am conjuring up ways to make this advice more useful than simply just substituting away from counting your blessings. I'll get back to you when I think I have something useful.