There isn't any one of us who doesn't occasionally feel pangs of, "I wish that person were eternally suffering for not having me in their life." Usually, it's just kind of a rotten feeling like they've moved on and we're still hurting.
Anymore I like to think of it as, "I'm being careful and conscientious about whom I date, and they're out banging someone like a screen door in a hurricane as a coping mechanism." Which, while likely true, is still nothing more than a coping mechanism.
Once a long time ago I broke up with a long time girlfriend of 7 years. We didn't talk for a few weeks, and things for me were really kind of ugly. When we did talk, the first thing she said was, "I'm dating a real man now." Which, you know, that hurt quite a lot. My (amusing but sad that I had to retaliate) response was simply, "How nice for you. I was so happy to be rid of you that I went to Disney World to celebrate." Which, she didn't believe me, accused me of making things up... So I scanned the plane ticket stub from the flight I'd taken.
(Yes, I had really gone to DW to celebrate, and luckily for me I walked into the hotel w/ a banner reading "Welcome National College Cheerleading and Dance Squad Championships!"... Oddly enough the previous time I'd gone I'd run into HS cheerleaders invading Busch Gardens")
Boy was that ever a great vacation for me. 
Anyway, short version is. Make them go away, and work on simply being happy for you that the stress and hassle is gone for now.
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."