It's normal after a breakup to go over all the good memories of the relationship and dwell on everything you don't have anymore. It's also common to conveniently forget the reasons why you broke up or why the relationship just wasn't working out. For many singles who have recently gone through a breakup or are still hung up over their ex, these memories replay endlessly in their heads, reminding them everyday of what they're missing out on. Partially, it's why the 'business of love' is truly a booming one. If you still find yourself pining over a certain person but not able to be with them, it's time to take the initiative and really try to move one.
Remind yourself that yes, that person was great, but they weren't perfect, despite how they may seem in hindsight. This list will help remind you of their shortcomings and really humanize them. Were they cheap? Selfish? Quick to judge others? Rude to your friends? Inevitably there will be things about them that just annoyed you, and seeing it written down on paper like that can really help you understand that they shouldn't be put on a pedestal.
Sometimes it takes a meeting with them to gauge your feelings for the person. It may be that while you still may have feelings for them, it just doesn't feel "right" anymore. Call them up and see if you can have coffee or lunch, to just talk. It will hopefully give you some of the closure you'll need.
The more you can see them separate from you and with someone else, the more you will be able to let them go. If they are with someone else already, it will be easier. If they aren't, imagine what it would be like to see them with someone else and steel yourself for it. If you really care about them, you will want them to be happy, whether it be with you or with someone else.
The more options in life you have, the more you realize that you really are able to love more than one person. Going out and seeing other people and just making new friends is one of the great joys of life. Sitting at home and sulking over a past relationship wont make you any happier, so try to ensure that happiness for yourself. Surround yourself with positive, happy people and build new relationships. It will give you the confidence to not only get back into the dating game, but a support system that you can fall back on no matter what.
Take down the pictures, the albums and delete him or her off your Facebook. It's all about taking down all the reminders of your relationship so you don't keep dwelling on them. The saying, "out of sight, out of mind," really is a true one. Deleting their Facebook also makes sure you don't get the urge to stalk them via the web and see exactly who it is that's talking to them, or where they're going and with who, since that will drive you crazy. Start with a clean slate and realize that you were fine before the relationship, and you will be just great afterwards too.