It's hard to juts "get over it" when your girlfriend dumps you for another man. It's not like things got bad, you mistreated her, or you were cheating on her. None of the usual suspects that send girls running were present in your relationship. So, the fact that she dumped you for another guy has really turned your world upside down. Now that you don't know which end is up, you really have no idea what your next move should be.
The first thing that comes to mind at a time like this is a toss up between wanting to get your girlfriend back and wanting to get even with your girlfriend. Most guys in your position believe that it isn't really possible to get your ex back and so you move on without taking the chance. You have no idea how many men miss out on saving their relationships by not exploring this options. She may be with another man but patience almost always wins out in cases like this. You see rebound relationships are notoriously brief. This means that if you're willing to wait a little while the odds are good that she'll be free again before you know it.
Of course, you will want to make the most of your time and not just sit at home pining away for your lost love. In fact, the last thing you want your ex girlfriend to think is that you're suffering over her while she's with the other guy. No matter how much it hurts, you can never let it show. But there are some things you can do that will put you in a position of power when the time comes for you to make your move and try to get your ex back.
The first thing you want to do, and perhaps the most important, is to give your self-esteem a bit of a boost. Do a quick self-evaluation and identify the areas of your life in which you are the least assured of yourself then take measures to correct them. When you feel more confident in yourself she'll find you that much more attractive.
The next thing you need to do is let go of things that happened between the two of you in the past. You don't have to forget them. You don't have to forget her. But you do NEED to stop letting slights and hurts of the past have power over you today. They will interfere with your relationship and your efforts to get your girlfriend back if you don't let them go.