Swedel, your posts remind me of some conversations I have had recently. But first, let me say that I am not yet a mono in a poly relationship -- I am someone who was married for 10+ years (totally mono), now getting divorced (not my choice), am just embarking on a new relationship, and currently investigating polyamory to see if it's right for me, although I am not yet involved with more than one person.
So, regarding jealousy, I have long considered myself to be someone who isn't usually very jealous -- with a caveat. In my relationships, which have always been monogamous, whenever I've had complete trust in someone's faithfulness to me, it never bothered me if they flirted with someone or visited an old girlfriend, things like that. Whenever I have gotten jealous, it's usually because I have doubts and insecurities, or if the person I'm with has lied to me or been deceptive in some way. So, for me, the degree of jealousy I experience is closely related to trust.
Recently, I've begun seeing someone. This is the first relationship since my husband up and left me (yes, I'm still pissed about that -- I've only been separated since June). Anyway, this new guy and I live kind of far from each other, and he's had some urgent issues and obligations to handle that have prevented us from being together much since we met. This past Sunday night, we talked and he told me he went to visit a woman friend of his that day to help her get settled into a new apartment. She lives at least twice as far from him than I do.
While he told me this, I felt myself immediately become jealous. I wanted to know if this was someone he was attracted to (I just thought this, I did not ask him). Now the funny thing is, he has told me he is a "one-woman man," but still, I had this reaction probably because the relationship is so new and I am still fairly insecure about it. Anyway, he went on to tell me that he had wished he could get out of it, but felt obligated to help her because she had helped him with something, and basically elaborated in such a way as to give me the clear message that this was just a friend and not someone he was interested in, even though I hadn't asked (a very mono maneuver). However, during the conversation, I experienced something that is unique for me. I somehow stepped back from the jealousy that had risen in me and examined it. I kind of asked myself why I was jealous, and I realized it wasn't this woman I was jealous about. I was jealous of the fact that he had traveled so far to give his time and attention to someone else when we have had so little opportunities to get together. I was a little hurt that I wasn't the first choice for who he would spend the day with, even though I had a really bad cold and wasn't feeling up for anything other than staying bundled up under the covers with cough drops and a box of tissues! I realized my jealousy stemmed out of my loneliness, feeling a little sorry for myself, and wishing someone wanted to be with me. For me, it's much easier let myself feel jealous than it is to feel lonely. I know I am much more willing to get caught up in the emotional drama that is stirred up by jealousy than to just sit and feel lonely or whatever else is underneath it.
I have read recently that jealousy is usually a conglomerate of various emotions, so maybe yours is also what's on top of other feelings that you are perhaps less willing to look at. It's something to consider.
The other thing that happened to me recently is that I got in touch with an old friend with whom I corresponded years ago (using actual pen-and-ink letters, not email! Anyone remember those?). We are catching up with each others' lives after 20 years' distance, and I told him how I have good days and bad days in dealing with my marriage ending. I was not having a good day when I wrote this email to him. Anyway, he responded with something that has really been food for thought. He wrote: "Your job is to love yourself unconditionally and energetically disconnect your projections from your partner. We always project ourselves onto our partners and when we lose our partners - unless we reclaim those facets of ourself we have delegated to the other - the feeling is as if our very selves, our souls are being sundered." And it occurs to me that this is very true.
I think people often want the other person to fulfill something in us, and we do somehow project something onto them -- perhaps it is indeed an aspect of ourselves that we project. You have stated that you know she loves you but you feel "cheated" and possessive. When you feel jealousy about your girlfriend's relationship with her husband, is there a part of you that you feel is being neglected, hurt, or that you're losing? I think that it is important to become aware of things like this and look clearly at the dynamics of a relationship, whether poly or mono, and at the feelings of jealousy themselves. When I stepped back from my jealousy to look at it, I was surprised at the sense of calm I experienced while doing so. We don't need to get wrapped up in our emotions, we can actually observe them. As I read more and more about polyamory and consider what it would be like to be involved in such a relationship, I think becoming clear about who one is, what one wants, the mix of emotions we experience, and what needs one hopes our partners will fulfill, would help deal with any jealousy (and other uncomfortable feelings) that comes up.