You have to find out why she left in the first place so that you can rectify the situation. Depending on how great your relationship was with your girlfriend she may be willing to spill out where you went wrong. On the other hand some girls don't have the patience to go through all that and so you will have to find out your faults on your own.

A female loves to be the center of attention. So one place you could start analyzing is how much time and effort did you put into the relationship to make it work. Some girls are shy and secretive so you may think that everything was OK but you girlfriend just kept putting up with all your wrongs. So think carefully, did you give her all the attention she deserves or did you just take everything for granted?

How well did you communicate with your ex. Communication is not just talking, you have to spend quality time with your girlfriend so that she feel that you truly care for her. Share your experiences and give an opening ears to hers. NEVER shun her away even if she wants to talk about something you don't like.

Be supportive to your girlfriend. Don't assume anything, if you see her have a passion for something help her to follow her dreams. Do to here as you would like her to do to you.

Once you can put out a little effort that you will change your bad habits and become more caring then you will be able to get your ex girlfriend back.