I grew up as someone who -never- got jealous, and didn't "get" jealousy. To my dismay, I've found through experience that certain types of problems in a relationship are breeding grounds for jealousy. Not being sexual often enough, or always having to initiate, or being turned down a lot, are what creates that for me.
Compersion (happiness at your partner's happiness with other folks, roughly) is a lovely thing, but (in my experience it rarely happens when you aren't feeling happy and satisfied with your own relationship with your partner, especially about sex.
Things that help:
- -Focusing on the issues in the relationship, and making sure that your partner also thinks that they are issues and worth fixing, and agreeing on steps to take to work on them. Feeling like my partner -cares- that I'm not happy with our sex life is a huge mellower of jealousy.
Barring that, which is awfully hard:
- - Get to know your partner's partners. It helps me immensely when these are people I like.
- - Refrain from asking for, or getting, any details of what your partner is doing with other folks. I'm not suggesting dishonesty here, but when i'm on your shoes, the last thing i need is my partner to paint me a picture of him playing with a given other person. (I use 'him' because i've only had this issue in relationships with men, for whatever reason.)
- - Have fun. Remember (and/or pretend), for a while that these other partners are NOT the reason you are unhappy with your relationship. Do things that are much fun for you when you aren't with your partner. Tell your partner about them. Don't let your happiness be based on what happens day to day in your relationship.
- - Decide, for yourself, how important this issue is for you. Try to keep that perspective. For me, I have this problem in my marriage, and I knew this problem was there (well) before we got married. I try to remind myself, when this comes up, that there are many many other reasons I married him, and that if this is the worst problem we have (which it generally is), we're still doing damn well.
Good luck with it!