I think GalaGirl and Magdlyn made very good points and have more experience with poly relationships than I do. Although I'm basically a newbie at poly relationships, I'm not at relationships in general. For any relationship to work including friendships, work , family romantic etc, there needs to be pardon the cliche, a "win/win" situation. This doesn't mean that everything will always be equal, but the idea is that everyone is getting enough of their needs met that they're basically happy with things.
". . . she broke up with this girlfriend because they had different ideals, but she made a point to tell me that it didn't mean we were exclusive again. she ended up wanting to get back together with this girl, but this time everything was more honest. we decided upon a polyamorous relationship."
Did you decide on a poly relationship or did she? Did you sit down and discuss it at length, how it would work, ground rules etc? Did her girlfriend decide on a poly relationship?
". . . it's been difficult. the other girl gets jealous of me & the fact that i live with her & i've known her longer. . . and i'm jealous because she seems to be more excited about her relationship with this girl." ". . . it's just the jealousy that rears its ugly head when they're together & i'm alone."
Regardless of whether you and her girlfriend "are friends and get along well" it sounds more like you both are acquiescing to the wishes of your mutual girlfriend and are going along to get along rather than being enthusiastic partners in a poly relationship.
". . . when i bring up the possibility of me having someone else she says she would break up with me because she's afraid of losing me & wants to leave before she's left. she's afraid that i'll fall in love with someone else, even though she's in love with someone besides me. she thinks its different because she's dating a guy and a girl, and i would be dating 2 girls."
Sorry but I have to call BS on this one, the gender is immaterial. Simply put, she wants it both ways. She wants to be poly as long as she makes the rules and gets what she wants. I imagine if her girlfriend wanted to start seeing someone else as well, your girlfriend wouldn't be digging that either.
This does not seem like a win/win/win situation, it seems like a win situation for your girlfriend.
All relationships require compromise, mutual respect and consideration for each other.
It gets exponentially more complex with more people, but the same principles apply. Unless you have a foundation of trust and are secure with one another, jealousy will always be lurking in the background.
Ultimately though, you have to decide if being in this relationship is worth the pain. I'm wary of giving advice, but if you're really motivated to stay in this, then I think that the three of you need to be completely honest about your feelings, what you each need and how this will work for all of you.
Love may be an infinite resource, but time is not.