You have lost your ex girlfriend to a breakup and it's driving you crazy because you miss her so much. All you can think is I want my ex girlfriend back. I know how you feel, the two of you have been together for awhile and you thought things were going great. Then something happened and she left.
You need to figure out what happened to make the two of you split up. It's time to look at yourself to see if it was you or some outside force that separated you. If you find yourself at fault, you need to suck it up and apologize for your actions. If you want to get her back you must do this.
Next think about letting her alone for a couple weeks to get over the breakup and you need to do this with yourself also. If your still emotional about it your not thinking clearly and making mistakes. Get past your heartache so you can move on.
Now, are you sure she wants you back? You should be able to tell if you have a chance when you look at what caused the breakup in the first place. If it was over something inconsequential you should have no problem at all. If it was something more serious, especially if another girl was involved you have your work cut out for you.
After a couple of weeks you may be able to initiate a little conversation with her and get more of a feel. Try to set up a date at a neutral location for some coffee and talk without arguing. This would be a good first step at getting back with her.