What do you do if your boyfriend left you for another girl? This kind of thing happens many times, and many times the girl gets her man back. This can be very upsetting that a guy would do this but there must be reason behind it. What you need to do is find the exact reason and overcome it.
While you are trying to find the exact reason why he left you for another girl, you need to resist chasing after him to get him back. This tactic will only push him further from you and make it more difficult to get him back. Don't worry about the other girl either, this may not be a serious relationship and one of them may dump the other soon enough to your advantage.
As to why he left you? Could you have been putting some pressure on him to get a little more serious or maybe you were talking marriage? A lot of guys are just not ready to take a relationship to that level because they enjoy the freedom they have and fear being tied down. To overcome this you could tell him you realize you were being pushy with him and he doesn't need to worry about you pressuring him. Things will reach the next level on their own when the time is right.
Another possibility as to why he left you could be something you have done or not done. Sometimes people can be annoying without realizing it, and others could be irritated enough to not want to be around. Could you have some bad habit that fits this situation? Hopefully your boyfriend had talked to you about this and tried to work something out before he left you, if not he is as much to blame. If he really cared about you he would have tried to work it out with you early on to save your relationship.