For privacy sake, I'm changing names into cheeses.
So I've been with my nesting partner (Cheddar) for about 2ish years, living together for most of it. She's wonderful, she's thoughtful, and she means the world to me.
About 6 months ago I met someone really cool (Swiss). We hit it off immediately and things have been great, except for one thing. Unfortunately I happened to meet her during a time when my relationship with Cheddar had a fair bit of turmoil, so unsurprisingly Cheddar reacted with a lot of insecurity and jealousy. The first night I hung out with Swiss we ended up getting in a huge fight. From there on, anytime I even mentioned Swiss all the energy was sucked out of the room.
I did my best to make sure I was moving forward with Swiss slowly, and did my best to try and bear Cheddar's feelings in mind. She still felt like I was putting more effort into this new relationship than I to my relationship with her. To her credit, I was absolutely far from perfect. I definitely didn't communicate how things were going between me and Swiss very well, to the point that Cheddar felt like I had stepped over a big boundary.
Fast forward to about 3 weeks ago: I've been head over heels in love with Swiss since July, but had been putting off prompting the partner conversation for the sake of Cheddar's feelings. It was my last chance to see Swiss for over a month, because she was about to do a bunch of traveling, and I decided I had put things off for long enough.
I tell Cheddar I'm going to ask Swiss to be my partner that night, and Cheddar reacts super negatively. She says something to the effect of "We're moving soon, your work schedule just changed dramatically and now I have to get used to having a metamore on top of that? It feels like too much is changing all at once."
To her credit, Cheddar later texted me to encourage me to have the conversation anyway, but by then I had already decided not to. "After all, even if Cheddar gives her blessing it's still going to hurt" I thought "Better just wait till Swiss is done traveling. Cheddar is right, and in her position I probably wouldn't like all of those changes all at once either."
Fast forward once again, but to last week. We're in the new apartment and slowly unpacking. Swiss has been out of town for awhile, and still has a week or two before I'll get to see her in person. Cheddar has been seeing someone (Gouda) for a couple of months. One night Gouda tells Cheddar "we need to talk". They hang out next chance they get, and Cheddar comes home to tell me "So, you have a new metamore."
It fucking sucks, and the timing could not be worse. I feel like I've just been consumed with jealousy. Watching her do all the things I've been holding myself back from for months: Coming home with hoodies, or hickeys. Spending entire days with the new partner. Nothing unreasonable in and of itself, but all stuff I've denied myself with Swiss for the sake of Cheddar's feelings. In the meantime I won't even get to see Swiss in person for another week. It certainly doesn't help, that because of my new work schedule I've had a lot less time to spend with Cheddar, and a lot of what I do have has coincided with her only opportunities to hang out with Gouda.
Last night we talked about it and Cheddar asked me "Do you want me to put things on hold, atleast till Swiss gets back?"
It feels like I'm being so unfair, but I said yes. Like, why should Cheddar have to put her relationship on hold, just because my other romantic interest is out of town? A lot of the things I've been holding myself back from weren't even explicitly requested by Cheddar, just the sort of thing that usually sets off her jealousy.
This morning Cheddar told Gouda "I need to take a step back for a little while for my partner's sake" and apparently Gouda didn't take it very well. She isn't talking to Cheddar at all at the moment. I feel like a fucking monster.
I don't know. Am I being unreasonable? Am I being unfair?
Asking Cheddar to put things on hold with Gouda doesn't help anything, but you have every right to be hurt, and absolutely need to have that discussion.
The issue isn't that Cheddar has a new partner and thus you have a new metamour, the issue is that you took things slowly with Swiss due to Cheddar needing time and space to process it, only to then not be given the same consideration or opportunity for discussion from her. And that's what needs to be talked about and addressed.
Asking her to put things on hold won't fix the problem of you not being considered or consulted, all it does is turn it in to a petty revenge thing, and create more problems (as you've seen).
Give her your blessing to keep things going with Gouda, but also, let her know how much she hurt you when she didn't give you the same consideration she asked you to give her. If she takes that onboard and treats it seriously, then you'll get through this. If she is dismissive of that, then the issue is in your relationship with her, not in her relationship with Gouda
I feel like you basically hit the nail on the head. Things are definitely more complicated than I was able to portray in my post. But I definitely hope this is how I'm able to move forward.