1. You're lonely.

Put simply, one of the main reasons you're not letting go of a past relationship is because you're lonely right now, said Erika Ettin, a relationship coach and founder of A Little Nudge.

"Rather than pining over someone who wasn't right for you, focus on yourself," she said. "Get back into hobbies you used to do, treat yourself to something that makes you happy, and get back in touch with friends who you haven't seen in a while. It's much better to be single than to be with the wrong person."