The conservative answer is that it takes up to six months to heal from a breakup. For those of you that don't like the sound of that, though, follow some simple steps. They should bring you closer to getting past this old relationship and prevent you from making the same mistakes in the future.
Cry Me a River
In the end, a breakup is a change, and big changes are difficult in almost any regard. Your ex knew you inside and out and it makes sense that breaking up with him or her could be overwhelming. Ending a relationship may also be the first time you experience a significant loss. Be patient -- people go through all the stages of grieving in a breakup: anger, denial, bargaining, depression, and acceptance. This is not the end of the world, though. Just don't hold it all inside or your feelings will get worse. Cry if you feel like it. Allowing yourself to feel sad will prove to be an important part of the total therapeutic process.
If after six weeks, though, you are still having severe depressive symptoms, and/or engaging in behaviors that are potentially harmful to you that you cannot control (like abusing alcohol and drugs or self-abuse), it's time to seek professional help. A good therapist can help you get over this hurdle in your life and lead you to find ways not to repeat this kind of relationship again.
Break Out the Vinyl
A little Cure, Sinead or Alanis never hurt anyone. You have the right to feel some self-pity and music can put you in the mood to do some melancholy reflection on your life.
School Yourself on Yourself
Being alone is good for you. Now that you're single again, you have the chance to regain a sense of yourself as a separate and stronger person. You can focus on the basic things you need to feel whole and fulfilled. Look realistically at your ex and see what you can learn from that relationship. Most people, unfortunately, idealize the previous relationship and look for somebody very similar only to end up making many of the same mistakes. If you don't want to go through this again, be as realistic as you can about the type of person you want to be with and what you are looking for in a relationship.