If you are looking to Get Back an ex Girlfriend you need to look first at which were the causes for your breakup, because if you don't know them it will be very difficult to sustain a healthy relationship.
How To Know The Causes of A Breakup
There are only two possible causes for a breakup. The first one is based upon factors that you couldn't control in any way, but you can work to mitigate. This for example could be the different values you have with your girlfriend, relatives that put obstacles to your relationship, critisism from people, etc.
Although you could not control those, you should know which they are in order to neutralize their effect on your relationship.
The other cause for a breakup is based upon your personality. This is the tricky one because you will not be able to spot these errors by yourself without learning a lot about relationships in general. The correct way to behave should be natural, but modern society has made people immature and because of that, they didn't developed the correct personality to have a strong and healthy relationship.
Fortunately you could learn anything you are missing and once you know all the causes of your breakup, you could fix them one by one (or mitigate them if they were external factors) and only them try to reunite with your ex girlfriend.
You will see that following this process will give you the best chances to Get Back an ex Girlfriend. You won't regret!.