Having your boyfriend suddenly dump you can be very painful, but finding out that he left you for another woman is almost more than you can take. It is humiliating as well as being heart breaking. You thought he loved you and then you realize he might have been seeing her all along. That makes you angry as well and you could be considering getting revenge by dating his best friend.
However before you do anything that you may later regret, you need to look at what happened without being so emotional. If you still love your ex boyfriend and want him back, you must consider his side of the breakup too. It takes two people working together in order to make a relationship happy and long lasting. It also take two people to tear a relationship apart.
It might be hard to face, but when a man leaves you for another woman, he is usually looking for something that he finds lacking in your relationship. Perhaps you were aware of some of his feelings and chose to ignore them. Now that he is gone, you have to face up to these problems and figure out how to fix them. Just calling him and saying you are sorry won't do it, you have to show him that things are different.
You might have let your looks go a bit and became sloppy in the way you dress. Have you gained weight and thought he didn't notice? Your ex boyfriend might have said something about these things in a joking way, but you should have taken him seriously. Since you didn't, you don't want to do anything that will make him any angrier with you.
Your ex boyfriend's new romance could be of the rebound variety and these seldom last. Don't consider the woman he is with as competition and try to tell him of her shortcomings. Instead, treat both of them friendly if you meet and work on your own shortcomings in the mean time. Having your ex boyfriend still considers you as a friend will make communication with him will be much easier.
Showing your ex that he can trust you could make you the first one he will turn to when the new romance begins to sour. When your ex boyfriend starts crying on your shoulder and telling you that he is disappointed with his new love, you have to remain neutral. Saying nasty things about her may make him want to defend her. Be patient and let him talk while you are being a good listener. Soon he will dump her and you will be right there for him to turn to.
Your ex boyfriend will think of all of your good points and seeing you the way you were when he first met you will make him fall in love with you again and come crawling back asking you to forgive him. Just remember what has happened and don't get so comfortable in your relationship that you think you don't have to keep working at it.