I try to look at poly in terms of the health level of each individual relationship rather than the act of being poly itself. I do think that of course there can be an overlap... losing yourself in one person can create damage in another relationship, for example. But if a relationship already has cracks, poly (like anything else) can either highlight the cracks or help to blend them together. I don't feel that poly itself helps a relationship, but the positive things that can come from it, such as increased awareness of oneself, communication, freedom, strength, etc. However, such things can be achieved in monogamous relationships too.
I really don't think that poly is a magic pill that will keep couples together. Most people I know are monogamous and I've seen most of them go through breakups and divorces. Breakups are about a lack of compatibility / giving up / being unhappy / something not working - I don't think it makes a difference whether one is poly or mono.
However, I do hear you and it sounds to me that you have struggled so much with jealousy that you are exhausted, perhaps scared, and perhaps not wanting to carry on facing what feels like an uphill struggle. I absolutely understand that.
After browsing through some of your old posts, I wonder if part of your feeling is that you are 'free' of hubby, Nudge is 'free' of his wife, who you seemed to struggle with and perhaps saw as the 'enemy' for a while there, and now you're seeing an opportunity to have each other all to yourselves? Though I also see that you say being mono with Nudge isn't an option.
It may be true that you only became poly to avoid a crumbling marriage. It's ok if you did. You were trying what you wanted to try at the time. I've come across various OKCupid profiles on my trawls where the owners of the profiles have said that they have tried both mono and poly and would consider either in the future. For them, it depends on trying things out.
I do empathise with your poly struggles. Poly can be difficult for me too, so I understand.
I can't be an advocate for poly and I can't be an advocate for monogamy. In past monogamous relationships, I have experienced a lot of cheating and mistrust. I was plagued by jealousy when I was mono. Having the 'security' of monogamy meant nothing in the end. In poly, I struggle with control, jealousy and insecurity. What that tells me, however, is that the problem is with me, not with the relationship model I choose. If I want to trust a mono or poly partner, I have to work on my own trust issues, and so forth. I think it's about knowing your own boundaries. Just food for thought.
Whether you continue to be poly is really down to you and you alone. You have to ask yourself what, if any, values ring true to you about poly. For me personally, it's about possession - I just can't believe in owning another person by controlling their heart. It's their heart, not mine. I certainly don't feel like I was born poly or am 'wired' that way. My GF on the other hand believes that she was born to be poly and that she's wired that way. It's different for everyone.
Also, I feel that whether poly or mono works is down to how well two people work together. For instance, I find poly tons easier when I have a partner who is extremely empathetic, sensitive, kind, committed, honest and wants to spend quality time together. On the other hand, I find poly extremely difficult if I have a partner that doesn't consider my feelings (different to bowing down to my feelings), that lies or omits, that can't balance well, that lacks empathy, and that doesn't care about quality time together because they are chasing the new person. Do you feel like your boyfriend offers practically everything you want in a partner, but feel stuck because he wants to be poly? Do you struggle with poly despite him dealing with it wonderfully?
Overall, there is nothing at all wrong with deciding that you don't want to be poly any more. If you do, or are unsure, I would suggest thinking about what you identify with in poly, perhaps reading some books (I really like Polyamory Roadmaps - I'm not a fan of Ethical Slut) and articles, and give yourself time to think it through.
Me: (30f) open poly
Serious long-distance relationship with GF (40f)
Casual FWB with Descartes (27f)
“Peace comes from within. Do not seek it without." ~ Buddha