There's always a reason as to why serious relationships may end. Its easy to romanticize the time you had together and make yourself believe she was the best you ever had. But when you dig deep and reflect, there are always signs as to why it ended -- arguing, lying, cheating, disagreements, growing apart, and many times just not compatible, but that's tough to admit. But if you try to revive a dead relationships, the cause of death may still be cancerous no matter how hard you tell yourself that it will be different.
The only way I see an old relationship can ever truly live again is with lots and lots of time for personal growth and evolution. The two people you were did not work. Just accept it. But down the line, if you two have grown significantly on your own and happen to reunite, a new romantic future might stand a chance. To be honest, you'll never get your old relationship back but it can possibly see life in a new body. However, as you evolve by yourself, you might find that you've outgrown her and will no longer want her back.Either way, don't make any hasty decisions. Utilize your guy friends. They help tremendously. Also, see Placeholders: The "In-Between" Girlfriends -- it might help)
