No amount of reassurance works because it is not about what the other person is doing, and even if someone does cheat or spent their love/emotional energy elsewhere (always working, a lone hobby...) it still is not about them---

The danger in jealousy is it asserts itself as if it must be assuaged to ensure survival (sound nuts?) and because it is so threatening, we can end up hurting other people or ourselves because of it-----------people kill out of jealousy. It is something that when out of control is dangerous (like depression, panic, anger, fear, -----) to one's self or others.
I don't know the answer. I was married 20yr and had "reasons" to feel jealous but those "reasons" were just a symptom of something else ---- maybe it is the thought, grown huge, of being out of control...of not knowing...never being able to Really Know...and the road is learning acceptance of not being in control in the large sense of the word... do work on it, I think recognizing it as part of you is a good place to start, everyone who can say "I am a jealous person" is ahead of anyone who simply goes after the other in rage and blames them. My theory is that it comes from someplace pre-verbal and that makes it really big and really scary and threatening because we cannot name its real source and put it in it's place.