I saw my new friend today for a second date, and we spent another solid chunk of time just talking, comforting each other over various challenges in our lives while eating breakfast, and holding hands. It was really lovely.
She said one thing that was really helpful for me. She has been in poly relationships for the past seven years, and said that her technique for feeling compersion and letting go of jealousy was this:
Imagine feeling happy for your partner for as long as you can possibly feel it (with regards to a situation that normally causes feelings of jealousy, envy, discomfort, etc). It may only be 3 seconds at first that you genuinely feel happy. Continually and gently push on that boundary of that feeling until you can feel that happiness for your partner for longer periods of time (10 minutes, an hour, etc; this process sounds like a very meditative practice to me). Eventually, happiness will be all that is left. (I think this sounds almost too simple, but why make a difficult internal process more challenging than it needs to be?) This practice reminds me a lot of Anapol's suggestion of learning to replace feelings of jealousy with feelings of compersion, so that you are replacing an old system with a new one.
I asked my friend how long it took her for her to feel compersion consistently and in a more complete sense, how long it took for that work to have an effect, how long before jealousy was a "funny" emotion to experience. She said about a year and a half.
It is really amazing to talk to people who have practiced polyamory so much longer than me because they can talk about their own difficult times with perspective; they are no longer in the thick of letting go of and reframing societal messages and norms around monogamy, possessiveness, and jealousy. They have moved past it. Knowing that this is possible is really empowering. I know people, like J, who have come into polyamory very naturally and find little difficulty in confronting jealousy, and I think this journey is its own unique and amazing thing to witness. Meeting this women, though, and hearing about how she, too, had a really difficult time with jealousy and now does not is really empowering for me to hear about.