How can I handle jealousy after I rejected a guy?

This guy was in love with me. He chased me for long time and I rejected him several times because I am in a relationship with another man. And I didn't find him the most attractive guy. He tried hard. But the last time he finally accepted my friendship. However, since then I feel very jealous of the women he 's dating. He is dating a lot and seems more extrovert than before. I feel so confused, because when I was rejecting him I wasn't supposed to feel jealous and here I am...

How can I handle jealousy after I rejected a guy?
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