Not just with jealousy, but being plagued by any unwanted emotion, I ask myself what am I afraid of? That is usually a great starting point. But don't take the first answer that comes to mind. So for example, maybe your answer is "I mad afraid he will leave me." Okay. Why are you afraid of that? Fear of being alone? Fear that it means you're not good enough? Fear of something else?
I don't have much issues with jealousy, because I am not afraid of someone wanting to be with someone different than me. I like myself. However, the unique me has a certain set of qualities (I can't have all of them). The other loves in my guy's life have different qualities. He enjoys mine and he enjoys hers. Why wouldn't I be okay with that?
And while it is counter-intuitive in a mono dominant culture permitting variety often elevates appreciation of everyone's qualities. As a simplistic example, my favorite color is purple. But if purple was the only color I ever saw? Now imagine seeing purple against a background of teal, yellow, orange. I love those color colors, but now purple is particularly outstanding.