No woman can stand to see her boyfriend dating another girl, not even an ex boyfriend, especially in she is still in love with him.
This is even more true if the break up was recent.
The question is, is all hope lost for you and your boyfriend? It all really depends on the situation and the context of things.
For example, was your boyfriend cheating on you with this girl? If this is the case, then that may have been his plan all along.
He decided to break up with you so that he could be with this other girl.
If you find yourself in this situation, then the relationship is probably not worth saving, and you would be better off just letting go.
However, in most situations, it is very possible to get your boyfriend back.
He may be just using this new girl as a way to make you jealous.
If he is doing this, then there is a good chance he is trying to get you back as well.
He just does not understand the right way to go about doing it, and decided to use this as a last ditch chance at doing it.
Do not fall for his lame tricks and give up on him by thinking that he has already moved on.
In fact, he is probably thinking about the day when he will get you back too.
The more likely situation that you will encounter is the rebound relationship.
He feels he has lost something by breaking up with you, and is doing everything he can to find it again.
He is trying to replace you with someone new.
Unfortunately for him and fortunately for you, he will probably not be able to find someone to replace you because of the feelings he still harbors for you.
In the end, he will realize that it is you that he wants.