Remember: healing is a process. You are bound to have setbacks along the way. This is just one of those setbacks.
Feel the sadness again. I know, I know! You've cried about your ex enough already. Unfortunately, discovering that he or she is dating somebody new is bound to reopen your wound, no matter how "over it" you thought you were. This is normal. Allow yourself to experience the pain once again. The good news is that this blow won't last nearly as long as the one right after the split.
Don't wallow. Sadness is only good as long as it helps to cleanse and heal your pain. The minute it becomes a mind trip of self-hatred, self-blame and self-pity, it is no longer productive. Try to stay on the positive side of this fine line.
Keep the details secret from yourself. Whether you hear the news straight from your ex's mouth or from somebody else, don't dig for dirt. When it comes to ex loves and their new loves, ignorance is bliss. The less you know, the happier you will be.
Chalk it up to fate. They say that timing is everything. They say that if two people are meant to be together, they will be together. If your ex has found a better match for him or herself, then you too will find a better match for yourself.
Do something new. Cut your hair. Take a dance class. Go on vacation. Now, more than ever, it's essential to do something--anything!--to get your energy moving in a new direction.
Flirt and date, but don't rebound. What's the difference, you ask? When you flirt and date, you open yourself up to new people and to the possibility of love. When you rebound, you immerse yourself immediately into a new relationship that isn't exactly right in an attempt (sometimes consciously, sometimes unconsciously) to distract yourself from your pain.
Look to the future. What exciting things lie on the horizon of your life? If there are none, create some.
Rekindle only if you really, REALLY mean it. Don't scramble back to your ex, professing your love, trying to reignite a snuffed flame simply because your ego is bruised. If hearing that your ex has a new love has made you sincerely reconsider your breakup and has made you truly realize how you can fix what went wrong between the two of you, by all means, tell your ex how you feel. But make sure that your intentions are pure before you jump. Otherwise, you're certain to cause intense agony and confusion for both of you.
Wish your ex well. Be happy that he or she has found love again. After all, this is somebody who has meant a lot in your life. You should want happiness and love for your ex. You will have happiness and love again too, so there is no need to be bitter.