The very first step for you is to realize that relationships just go wrong sometimes. It might have been your fault and it might not have been. There are a huge number of relationships that end over what seems like nothing. If you want to get back together the next step is to figure out what exactly went wrong.
Don't be one of those men who think that there is no way they can find out what went wrong. If you just give up on figuring in out the chances are very slim that you'll ever get back together. You should never give up. If you want to get your ex girlfriend back you shouldn't give up.
Go somewhere quiet where you can think. Think long and hard because this is good advise on how to get your ex girlfriend back that you will find. Take a pen and paper out and write down the things that come to your mind. Were there things that you definitely should have done different in your relationship? If you can change those things are they worth changing?
If you just can't think of anything you can go to your ex and talk to her about it. You might find that she's more than willing to tell you about the things that she thinks went wrong. Some couples won't be ready to talk to one another and that's fine. Do the best you can with your own assessment for now. When you have a better idea formulated in your mind you can start your strategy to get back together. Don't expect this to be easy!
Try and figure out what made you break up. This bears repeating because if you don't do it you never will. If you can fix this or start working on fixing it you can both see the good in your relationship.
Then, you need to think of a way to get back into your girlfriend's mind in a good light. Prove that you have changed. Show her with your actions as well as your words.
Finally, you need to reconnect. You can't expect everything to instantly go back to the way it was before you broke up. The best thing for you to do is take things slow. This can be a bit maddening at first, but as long as you have the love of your life back in your arms you should be able to cope.
Take this advise on how to get your ex girlfriend back. You will never know if you could have won her back if you don't try. Do everything you can because love is the most important thing in the world.