How To Deal When
Your first instinct may be to freak out as soon as you find out your ex has a new flame, but do your best to maintain your composure. Allowing yourself to fall apart will just send you into a downward spiral. Rather than wallow in it, use the news as a sign it's time to move on. Call some girlfriends and meet for dinner or coffee so you don't have to be alone. Talk about the situation with them, but then cut yourself off from over-thinking it.
If you end up hearing about what she does, what she looks like or how she dresses from mutual friends, the worst thing you can do is start comparing yourself to the new woman. Whether he's dating your physical opposite or your doppelganger, what your ex does now that you're not together is no concern of yours. Don't drive yourself crazy with the details about his new girlfriend.
If you and your ex are still connected via social networks, resist the urge to peek at his profile to see if he's posted pictures of his new flame. Seeing them together in some loved-up pose is just going to make you feel worse. Stay off of Facebook until you think you can handle the possibility of seeing them together.
Hearing that your ex has found someone new has the potential to send you into a panic. That's understandable, especially if you're not over him. But try not to torture yourself by going over your breakup with a fine-toothed comb just because you can't quite believe he's moved on. The two of you just weren't meant to be -- so do your best to move on, too.