The notion that jealousy is bad, is actually a relatively new notion, and one brought on by "civilized society". The truth is that it's actually unnatural to not be jealous. Jealousy is a natural instinct that arises when we feel threatened. Jealousy is an instinctual emotion that helped us survive in neanderthal days. Without that natural emotion, we wouldn't have found any reason to keep our mates near by to protect the family, make more offspring or bring in more resources.
The emotion of jealousy still serves the same purpose in today's age. It's just that we teach that it's not right to act on it anymore. There are more ways to provide resources, to create offspring and to protect the family without locking down a mate.
So when a common sense person knows that it's not "right" to be jealous, yet they still feel it, it can drive you nuts. It's because it's a natural emotion. The trick is to reminding yourself that the emotion is not as necessary as it once was, and that you can teach yourself to push that emotion aside. You'll still feel the twinge from time to time, which is natural, just don't let it eat you up.