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Re (from original post
"Neither of us have tried this in the past, and I'm not sure I'm handling it as well as I thought I would."
Emm; sounds more to me like *your fiancÚ*
isn't handling it as well as you thought (and he promised) he would.
If the only problem here were your struggles with jealousy, there's lots of good stuff out there to help analyze and deal with jealousy, e.g.:
Let us discuss the greeneye monster shall we?
How to slay the greeneyed beastie.
Jealousy, Envy, Insecurity, Etc.
How do you achieve compersion?
The Theory of Jealousy Management
The Practice of Jealousy Management
Jealousy and the Poly Family
Kathy Labriola: Unmasking the Green-Eyed Monster
BrenÚ Brown: the Power of Vulnerability
But you know, sometimes jealousy has causes that aren't internal to you at all; rather, it can be your brain's way of sounding the red alert and warning you, "Your romantic partner/s is/are treating you unfairly!"
So, is your fiancÚ treating you unfairly? Why yes he is. He's been lying to you, and seems to have a persistent history of lying that isn't about to end. I mean really? Enlisting his friends to cover for him? That's not just a lie, that's a conspiracy.
True I don't know his side of the story, but based on your side so far, I have to conclude that it's his behavior and attitude that need changing, not yours. You're doing fine. Here you are on Polyamory.com yet, trying to learn more about what polyamory is and how it works so you won't fear it so much -- and asking us what you're doing wrong, cause if there's a problem, it must be your fault, right?
Alas, the propensity to "be poly" [in this case read: sleep around behind your primary partner's back] doth not automatically a noble soul make. Some so-called polyamorists are just jerks who use the wonderful-sounding notion of polyamory (and polyamory can be wonderful if done right) as an excuse and a get-out-of-jail-free card. Don't assume that just because you're the "non-poly partner" automatically makes you the guilty party for shiz that goes wrong.
"He's told me that he'll break it off with her if I ask him to, but I know from previous conversations that he'll eventually be miserable again and will most likely find someone else to replace her."
So in essence, he's got you emotionally blackmailed. You can either go along with his current on-the-side fling, or you can look forward to him cheating on you again. You're not agreeing to poly. You're being logistically dragged into it.
I'd almost say leave, but I know it's a tough call when you've got kids. Just remember, sometimes it does a number on a kid's mind to see his/her parents struggling to get along and failing, especially when it's because only one of the parents (in this case you) is *really*
putting the effort into it. Bad example! Sometimes it's better for kids to see the dissed parent stand up for healthier boundaries and separate from the other parent if that's what it takes to maintain those boundaries. Kids hurt? Yes. But they were being hurt (at least mentally) anyway, and this way at least they know that when they grow up, they should stand up for their own boundaries.
If you do stay, at least tell him in no uncertain terms to shape up.
He promises you that he'll never fall in love with another woman again -- he'll merely sleep with W (or whoever). Who can make such a promise? Who can guarantee their heart will follow this or that strait and narrow course and never a forbidden longing feel? Sounds to me like he's lying ahead of time about the future. At the least, I think he's kidding himself. I don't think he can "stop himself from falling in love" just because he said he could (and would). Falling in love is like a force of Nature. There's virtually no way to stop it. Control your actions in response to it? Yes, you can do that. But to stop/control the feelings themselves ... that's a whopper of a proposition.
It seems obvious to me that you still love him and are in love with him, but that he's tearing you apart with his exploitative behavior. And it only worsens the pain to ask this, but you've gotta ask: Does he love you? Does his behavior show that he loves you?
Technically he hasn't been caught in a new lie yet, so it's understandable that you want to try and give him this additional chance. But I'll say this: Watch him like a hawk. He's not good at being honest. He's used to, and comfortable with, being a con artist. Dishonesty can be a habit, and he's got that habit. It won't be an easy habit for him to break, even if he honestly (!) tries. So ... careful ... careful ... careful. That is all.
I don't envy your position, you've got a fine line to walk here. I hope Polyamory.com can help.