Relationships are never easy and though it may look from the outside as if some poly relationships are simple, brilliant and sorted, I think that those relationships took a hell of a lot of soul searching and exploration before they reached that point.
I think the best way to tackle your predicament is to think of the two issues separately. A lot of people think about poly like... if you're not 100% happy with one partner, that's ok, because you have have multiple. You can't get everything from one person. I think of it like... would I choose to spend this amount of time and give this amount of myself if I were single? Or would I move on? I date secondaries who I wouldn't choose as girlfriends if I were single.... but I would choose them as casual partners, and so forth.
I say this because... in my opinion... being unhappy, especially with a primary partner, can cause a great deal of heartache and at the very least, make poly difficult. You find yourself craving the attention and new love of another, whilst wrapped in a fading-yet-familiar security blanket with your husband. But here's what I know.... if my girlfriend ever got to the point where we were mediocre and she was fully throwing herself into someone else... not only would I struggle more with poly; I wouldn't want to be there any more. It would break my heart.
So that makes me wonder about how your husband feels. And hey... relationship breakdowns are not usually one person's fault. Relationships break down because of a lack of compatibility and/or a lack of communication. If you've had other issues, they've obviously widen the cracks and in your heart, three years ago, you were looking for a safe way out.
Ok, so let's look at your husband first. I do not believe that marriages have to stay together. However, 12 years is a long time and family is important. Sometimes, the best way to understand what somebody means to you is to not have them around for a while. I don't know if that could work in your situation, due to having children. But if it was me, I'd be tempted to push myself to move out for a period of 6 months, if it was financially viable. Then see what life is like on your own (which is different from 'alone').
I know these measures are not always possible... and so, if you are to work out your feelings whilst remaining in the home... I say give it a shot. Start having weekly dates. See a marriage counsellor - a poly friendly one. As Schrodinger said; reignite the intimacy between you.
If reading that paragraph made you feel claustrophobic or resistant... listen to your gut. Is what you really want a platonic, close relationship with your husband, but romance with other people?
There are all kinds of different relationships and plenty of monogamous married couples live separate lives, even sleep in separate beds, whilst playing happy families in front of other people.
I actually live with my ex boyfriend in a studio apartment. I have a girlfriend, he is dating someone, yet we sleep in the same bed. Like you (perhaps?), I basically outgrew our relationship and needed to stretch my wings... but neither of us wanted to let our closeness go. This works for us... I say, make your own rules.
Onto your boyfriend...
Meanwhile, I had run into a roadblock with my boyfriend- I had decided I wanted to be with him instead of my husband and wanted to try to find a way for us to be together, or move closer to date more regularly and see if we'd work as a couple, not just long distance. He didn't want to. He wasn't comfortable with kids, and didn't want to settle down, and didn't want me to leave my husband. So we settled back into the triad, with me trying to accept this, and just be satisfied with what I could have of him.
Either your boyfriend truly is a person that you really would be better suited to in real life, or the romance of it has taken your brain to a place of escape. Sometimes, especially when face-to-face meetings and time together are short and far apart, we don't really know our loves as well as we think we do. But, we do get the support, sweetness, excitement, sex, etc. from them.... and this is going to be packaged up in a little bow that says... "wow... I prefer this to routine, predictable marriage".
For a start... take a look at your boyfriend's response to your thoughts on leaving your husband. What got you to the stage where you suggested it? Had you two ever talked about that before?
I'm not saying your boyfriend is wrong for not wanting to be with you exclusively, because of your children and whatever else. But... what I'm saying is.... is he really the man you think he is? Or is your mind projecting a reality with him as an escape from the humdrum and difficulties of long term marriage?
There was an issue with jealousy over my boyfriend as well, as whenever his ex almost came back into his life briefly, I had such a hard time I tried to break up with him but he talked me out of it. Then again when an old friend of his, and a submissive that he admitted he might like to play sexually with was supposed to move in with him temporarily when she lost her job and was looking to move to another city, I was in agony.
So.. if I have this right... your jealousy issues spiked after you suggested leaving your husband? Or was it before? Either way, to me, it's obvious why you are struggling. Yes, jealousy comes from insecurity which comes from self esteem, etc etc. Anyone who's been poly for three years should and would obviously already know that.
But.... what about relationship security? How CAN you feel secure when you possibly want something more than your bf does? Perhaps the demons in the back of your brain are telling you that he will find someone without children, or the type of marriage 'baggage' you have, and will leave you for them?
Well... it could happen. Or, it could happen, but he'd still stay with you. I really struggle with the idea of "trust in your partner.. believe they won't leave you... it's all in your mind". I think anyone could leave anyone; mono or poly. What concerns me more is ... "trust that my partner is being honest about what they want... what their commitment level is towards me... and where I fit in their life"
On the slightly brighter side... I am hoping that this little piece of anecdotal advice might help you -
When I met my girlfriend two years ago, she was on the brink of divorce with her husband. She gave him an ultimatum of divorce or poly... and he took the poly route. She threw herself into other relationships, wrapped in the security (financial included) of their 11 year marriage. They also have a three year old.
A lot of things led up to their difficulties. My girlfriend had moved and moved and moved so that her husband could follow his dream career. He'd squandered a lot of their money. They lost their dream house and moved into a tiny apartment, with about 1/5 of their income. He was diagnosed with OCD after the birth of their daughter and she has had depression for most of her life.
For the first year or so, poly gave her the freedom she craved, but pushed them further apart, because she basically neglected their relationship, instead of dealing with their issues. She met me, they eventually both accepted me into their family and now, two years later, their marriage is growing stronger and stronger again. She still contemplates whether they are meant to be together... and in all honesty, what they have is a long-standing, familiar, sometimes fun, friendship-based marriage that suits them for the very most part. And they are happy with that.
I think that you could spend some time reflecting on what kind of qualities you want in a long term partner, what you get from your husband and don't get... and the same with your boyfriend. There is no need to make a decision.. but I would definitely try to work out your feelings and desires, before trying to tackle any jealousy issues about your boyfriend.