It might be a matter of time. I don't think you can stop feeling jealousy immediately.
When you have these feelings, I think you need to step back, ask yourself why you feel that way, and see if the cause is reasonable or not. Reassure yourself. Once you feel better, it should be easier the next time you reassure yourself, until at the end, you might still feel the jealousy, but you should be able to nip it in the bud.
Your feelings are part of who you are. You need to accept them, because that's the only way to "fight" them. Don't fall into the trap of burying them or calling yourself a terrible person. Look at the feelings in a factual manner. "Okay, I feel this. Why? How do I fix it?"
Staying rational is the first step and the hardest one. Once you've done it, you can look at the whole thing clearly and analyse it.
At some point, you will need to let go of the fear of losing him, but that's probably going to be the hardest step. At some point you should reach some kind of "zen" point in which you'll "realise" that if he wants to leave you, he will. And that point is both terrifying and liberating, because you start feeling like there is nothing you can do, it's going to be 100% his decision. And therefore, you don't have to fight things that are happening. They are happening. You can focus on the part you do have control over, your relationship, and trust that he loves him, and therefore won't leave.
I'm not sure how clear I am, and I can't promise that it happens the same way for everyone... But for me, I'm a control freak, and someday something just clicked inside of me, and I realised I have no control over whether he stays with me or not. I mean of course, I can be good to him rather than horrible to him and that will affect his decision :P But his decisions and actions are out of my control.
And although it felt like I lost something there, I started letting go of jealousy as well. He wasn't mine, he was never mine, he was his and he was letting me have him. I had no claim on him and therefore I lost the possessiveness, the idea that something might be taking him from me. Because he wasn't mine, nothing was actually taking him from me.
I really hope I'm not making you feel worse here. For me it's still a little bit scary at time, but the idea that I don't have to monitor the relationship because it's NOT my responsibility, and I'm not the captain, and I'm not to blame for everything that happens, was really a weight off my shoulders.