Sociologists and psychologists have studied this question. It turn out that, in general, men have a more difficult time moving on. The reason why is because men tend to brood over the break-up and close themselves off. In other words, they don't talk about it. Women, on the other hand, generally have girlfriends that they feel comfortable sharing their feelings with. This sharing and communication helps the woman move forward---it is a release of sorts. The male, however,may not necessarily feel comfortable in sharing his feelings with his guy friends. Consequently, he remains "stuck."
This is why a man who was married, is more likely to want his wife back, whereas the woman feels differently about her ex-husband.
So in essence, her "talk therapy" with her friends helps her to let go of lingering attachments. Of course, there are exceptions both ways, but in general, sociologists have determined that women have the advantage in this area (of being able to."close the door" to the past.} Great question!