What GG said.
That said... I'm Mono, but in a Poly relationship because I value my relationship with my partner more, but I had to (painfully) let go of my desires for a traditional Mono relationship with him. I had to let go of what my future may look like (how do I envision retirement, how do I envision growing old with him).
What helped me was time and openness in communicating this all with him and his OSO, and being excruciatingly, painfully, honest (I had a LOT of resentment toward his OSO for "changing" him, when I should have also harbored some anger toward him at dangling the "marriage" carrot, and at myself for clinging so tightly to it). It was like mourning a death, to be honest, and it was NOT easy. I still find vestigial remnants of it here and there, as well, and those have to be dealt with as they come up.
What also helped me was pulling back and looking at things from the "30,000 foot level". Being married was no guarantee of what my future would look like, either (says the divorced girl who had no idea what to do with herself afterward).
One thing that is a continual issue for me (and your mileage may vary, but I want to give you a heads-up) is time. I need a certain amount of time with my partner in order to feel like I *am* building a future with him, or that I have a home with him. Due to various circumstances, we're not there yet, and it still feels like "dating" or "he's coming over" to me, rather than "he's coming home". One of my concerns is that it may never feel like "home", even when we're able to spend half-time together. If that happens, I'll need to honestly evaluate the relationship and see where we need to go. In the meantime, it's a good relationship, and I have hope, so onward and upward we go...
Good luck to you and all involved!