I'm sure you are in a lot of pain and feeling depressed about the situation. The pain can't be helped, but you can stop the depression. I assure you that I know how you are feeling right now. I have been dumped a few times myself. Honestly, no matter how many times you get dumped, the pain never eases up.
What you should do is take your mind off of your ex first. The best thing you need to do is to turn all your attention to YOU. Try to make yourself a better person during this time apart. Get a new haircut, buy new clothes, go to the gym - anything that gives you more confidence in yourself.
Fight off depression by re-connecting with your friends and family. Spend time with them doing things that YOU want to do. They will be your main emotional support at this trying time in your life.
Prove to your ex girlfriend that you are not going to allow the break up to destroy you. Take control of the situation. You should try to change the way you think and you can become more positive and bring good things out of the break up.
Don't spend your time wondering how to get your girl back. before that you need to get yourself back on track. I promise that once you do this, you will become much more attractive to other people, and this includes your girl. Then you can start focusing on ways to get your girl back.
A while ago, I read a very good book that helped me change my mindset and taught me how to get her back. Check it out, it might help you too.
Ways to Get Your Girl Back