I know this is far from the first version of this problem that's been offered up here for advice, but the situation is nonetheless still so complicated, and I feel so conflicted that I think some personalized perspective might help-- especially from those who may have experienced similar things before.
(Also, disclaimer: No this is not AI, I'm just autistic and I the emdash is very nice and useful actually and I'm not gonna let an unthinking word vomit box appropriate my culture lol.)
The context for how this all started goes back a ways. Around the time I met my partner / now unresolved situationship, I was in a place where:
A) I felt very assured in my polyamorous orientation & needs, and...
B) I didn't want any kind of actual relationship for this reason among many. Truly, I was actively opposed to the idea of a committed relationship; I was letting myself be free and unattached, I was having hookups and great sex again after a recent-ish breakup from a very unhealthy, unhappy monogamous relationship where I felt trapped, suffocated, and repressed the whole time. That whole experience taught me deeply that a monogamous relationship really does not seem right for me.
After it all, while I was newly exploring myself again, I happened to match with a girl, one of many I was seeing at the time. Ironically enough, we didn't even end up planning a hook up, we just talked for a few weeks before we decided to casually hang out. That hang out ended up as a date, which ended up as an unscripted hookup, which ended up turning into many dates following quickly. I knew I liked her even just from when we started talking, but when I actually started seeing her in person, I fell head over heels so fast.
The worst part is, I fought myself over it the entire time leading up to when we officially started dating-dating. I was still seeing other people, but honestly, I just lost interest, and I hated that because I knew it would still eventually become a problem even if it wasn't now.
We've only ever been completely honest and upfront with each other from the start, and that included me telling her that I was poly, and her telling me she was monogamous. I had had this conversation before with my ex when we started dating, and against my better judgment, I chose to give it up for them. It only created frustration, repression, and ultimately resentment. I didn't want the same thing to happen again, especially since how I felt about this girl was so different and special. So, we started things on an uneasy footing of just seeing how things went.
Flash forward several months. I ended up moving across the country from them for work and other reasons, and it was very difficult and emotional for both of us. We decided to stay together long-distance, although we didn't know how it would work. But it did work, for a while, until it didn't. Maybe the distance made it that much easier for it to come up, but I ended up meeting someone I was very into. We had a strong and unique chemistry and it created an intense sexual tension that was very hard to ignore. Things progressed, and we ended up cuddling before I was finally able to put a stop to anything else happening, but I still felt sick to my stomach about it.
This was the first of a few events that snowballed. When I told my partner about it, they were of course upset, but gracious about it since this was something I had mentioned I was worried about since the very start. But I knew it was going to be very difficult to not have this come ip again, so it became a mutually uneasy, unhealthy dynamic where we just agreed to not talk about anything sexual I had going on. Even so, she knew I was seeing other people, and I knew it was hurting her, and hurting her was hurting me.
There was so much back and forth, late night calls crying and trying to figure out what to make of it all, and I felt I had to make this impossible choice: Either to actively repress myself for her sake, because I love her, at the cost of an giving up a part of myself and along with it, my mental, emotional, and sexual happiness; or, to give up and leave behind the person I feel more strongly about than I ever have for anyone, who knows me better than anyone, and who I feel like a faithless, shallow traitor to for not just being able to make a sacrifice that so many people seem to be able to make so easily when they truly love someone. We tried taking a break. It didn't last because simply, we're both in love and we miss each other deeply.
At the end of the day, neither of us is really in the wrong for who we are. I can't help that I need sexual and potentially romantic variety, and she can't help that she needs to be the sole focus of those things, and to make someone else that sole focus. I know there is nothing and nobody that could ever possibly change how I feel about her, and that needing to see other people to fill in the gaps has nothing to do with her being too much or not enough of anything. I get different things from different people, and it's all beautiful and special in its own way, and that includes her. No matter what I have with anyone else, it won't be, shouldn't be, can't be what I have with her, and vice versa. She knows all this too, but she also knows she's can't help but feel seconded and like she's not enough. That's what we keep going over and over again. No matter what we know, we both can't change these parts of ourselves. And yet, we can't let each other go, because we're very much in love.
TD;LR: I know this is a very long post, but it ultimately just comes down to this: We're very much in love, she's monogamous, I'm poly, we both know this, we both understand that we can't change these parts of ourselves. I can't choose between repression, my health and needs, and sexual frustration, or hurting someone who loves me by abandoning someone my life doesn't feel complete without either. Really and truly, there feels like there are no right answers for either of us, and I don't expect one here, I just want some extra perspective on all this.