That's a hard one and thus especially worth looking at.

First off all: Open up for the feeling, don't fight it if it arises. It may be safer to access that feeling before it is an actual situation in your life.

Look at the dynamics in your body, how does it change your breathing, your energy level? Don't attach to the feeling, don't identify yourself with it. You are just observing. Be open for the idea that it is just a different state, it might feel even positive. Try not to resist. Resistance is fear and that creates the negative aspect of that type of energy.

Things you could look at: whats the basic fear behind jealousy?

Being alone: That bases on identification with another person or life circumstances, but remember that "you are" not more, not less. As a basis it also refers to the idea of separation - which is an illusion. We are all connected - on all levels of existence, even if you just look at it in a physical way.

Identification with the desires of your partner: His/her desires are not yours. You don't need to have these desires to be loved. You are loved for being who you are, not for being someone else.

Attachment to the partners body: it's physical. That's all. It doesn't keep you alive, nor does the physical make you happy - it's all in your mind. The thought pattern that the physical does make you happy triggers the emotion that you are happy in certain situations. But the physical is just a symbol for the happiness that is inside of you.

Attachment to the partners attention and devotion: again, this behavior is a symbol and works as a trigger for what is inside of you. If you can realize that you can experience a great deal of freedom, because what you find out is that you are fighting against yourself, against your own thought patterns that have no reason for being.

If you really loved someone, you would be happy with everything that makes this person happy, right? If having another partner makes this person happy, you in turn should be happy, too. If the person is more happy if he or she leaves you, you should be happy as well. If it doesn't, you can call yourself lucky to come across this disturbing thought pattern that you now can work on.

Still, it does kill me at times, but the goal "peace of mind" is worth the hassle.

Safe & happy journey to everyone!