When I opened the relationship with my mono partner, he took it hard and personally. I wasn't interested in anyone in particular so I was fine with going slowly. We rediscussed it a few times and he seemed to grow more comfortable about it as time passed.
Fast-forward two years, he got interested in someone and told me he was poly all along, and mentioned examples from his high-school years, etc. I had a crush at the time (I had several over the years but wanted to wait until he was comfortable before acting on anything) and I started helping him pursue the woman he liked. I also tried to get closer to my crush by nothing came out of it, however I met Seamus, my current boyfriend, a bit afterwards.
Now I've been separated from Raga, my husband, for a year and a half. We had many issues, mostly trust, and a couple of months after the break up he told me he was mono, had always been, and was pretending for my sake. He seemed resentful that I didn't realise he was acting and that he was actually miserable.
Thinking back, I still remember him as the happiest he'd been in our whole relationship at the time he pursued that woman, and at the time I started dating Seamus. I have to admit, I still don't know if he was pretending then, or if he got bitter after the fact and changed history. Either way, I definitely think that if something went wrong with polyamory, it was the trust issue and a lack of communication. I always tried to communicate with him, but it became obvious over the relationship and after it ended that he tended to tell me what he thought I wanted to hear.
I don't know what I should have done differently. I was obviously intimidating, sometimes it felt like he was actually afraid of me. I hated it. Bottom line is, my advice for you to get out of my experience is to communicate more, and make sure everyone is comfortable, and double and triple check. And for everyone to be honest with their partners.
It's fine if your mono partner is struggling. It happens. It's normal. But they need to let you know so you can deal with it. So make sure he's aware of that. Or they, if both are mono. And I would also say I wish their had been more communication between my husband and boyfriend. They both wanted to hang out and chat in theory, but never really did, and I had to actually hold their hands and make them spend time together. And then again, only when I was around. I think things would have gone better if they had gotten to know each other more.
This being said, ultimately I don't think the end of the relationship was due to polyamory, and I'm not even sure if the polyamory made it last more or less. Sometimes I think maybe polyamory allowed me to see the problems better. But then again I remember the two years we were open but nothing happened, and I hung in there thinking it would get better. If I hadn't had that hope, would we have broken up before then? That's possible too.