If you've just managed to get yourself together and build up the courage to fight to rebuild a relationship with your ex, it can be a harsh and debilitating blow to discover him in a new relationship with another woman...but is it really as discouraging as it feels? The reality is, rebound relationships can actually be a good sign, except for the very rare cases that marriage is already on the books. 95% of rebounds are very short-lived, and are really just ways for your ex to deal with being alone and reassuring himself that he can make it without you in his life.
It actually wouldn't even've been too bad if YOU'd had a rebound of your own...it builds your confidence a lot and can work wonders on helping you deal with the heartache...but they're often very temporary and nothing can erase your memories of the one you love...he's not necessarily over you just because he's with someone else now. It may mean he's still TRYING to get over you.
This isn't always the case, but you'll usually find that it is. A great many couples have resolved their differences even after seeing other people, and often these experiences merely served to show them that the few problems and fights they had weren't enough to forsake all the great and wonderful aspects of the relationship. Everybody has fights and problems...but rarely are they so bad that it's simply impossible to recover. Many times, playing the field a little bit will help you realize that, so don't fret too much if your ex is dating.