My husband and I have been together for 10 years. We have kids, cars, a home, and careers built together. We've been off and on exploring non monogamy in many forms for about 7 of these years. We've tried dating together and it didn't work out for multiple reasons, but the main thing we got out of that experience is that my husband (M) has big time NRE. Almost to the detriment of our relationship. I move much slower and need to form emotional connections before I can feel romantically involved, and he falls in love on the first date. So dating together, we quickly learned, was not a good idea. Fast forward to the present. M has said since our last experience that he doesn't want to date for now and he wants me to find somebody I can genuinely bond with by myself (it hasn't happened by the way). He meets a coworker and thinks she (C) and I will hit it off. He introduces us, we get along, and we all talk about boundaries. C says she just got out of a long term relationship and doesn't want anything serious. Only casual and she wants to see both of us in a FWB situation. Sounds fine, we're all on board... But then almost immediately they are saying, "I love you". And neither of them told me about it until afterwards. C says she really likes me and wants to get to know me too, but hardly responds to my texts and doesn't make time to spend with me. I haven't spent legitimate time with her in 2 months, meanwhile they have spent numerous evenings and nights together. I feel like I'm being crazy and ridiculous for being so frustrated. We all agreed on casual and they broke that almost immediately. They said they wanted to slow things down, but are still seeing each other multiple times a week while I hardly hear a word from her most days. I am constantly hearing important details through the grapevine of M. C says she never used me to get to M but that's how it feels. I've made my feelings known to both of them and still feel like nothing has changed or been done to address the inequity in our dynamic. I don't want to have veto power or couples privilege but I feel like I'm constantly being left out when this was never meant to be serious in the first place. Obviously this is just a very quick summary but I don't know what to do or where to go with this anymore. M is trying his best to keep the peace everywhere and she and I just keep butting heads. C wants a stable, primary relationship with marriage and kids like M and I have, and I feel like she realized she can't have that with M like she wanted so now things are just ugly and weird. I feel played. Any help or advice is appreciated.
The first thing that jumps out at me is that it doesn't really seem like you have decided how non-monogamy fits long term in your life. I feel like if you start with that, that may make the different ways you do relationships not something that causes so much anxiety for you. Because at the end of the day, you aren't wrong for wanting to go slow and he's not wrong for wanting to go fast. People do different things at different paces and maybe M wanted to "go slow" for you, but I don't think trying to control the speed at which he forms emotional attractions or attachments probably isn't the way to go.
From the beginning, you feel neglected in this relationship and while C's long term goals might be something that stokes your nervous systems, really C is just the salt inside of the primary wound here. Just because your husband develops romantic attachments quicker than you doesn't really give him the excuse to neglect any relationships -- however if you don't ever make it clear between you how non-monogamy is going to work along with your kids and everything else, then maybe it's less clear to him when it's neglect and when it's not.
Furthermore, despite everyone's assumption that couples who live together have all of the power in every way, my personal experience, in multiple relationships, is that when someone lives with you, it's very easy to assume that time spent in the same house is quality time. You share a load of responsibilities that weigh on your relationship and communication that C doesn't have to worry about with M.
While I haven't conducted any wide survey of the polyamory community, it wouldn't surprise me based on my own experiences, doing this column, and speaking with others, that people who share a living space with their partners might chronically feel like they get all of the negative aspects of managing a relationship while "secondaries" or people who don't share that space, get a lot of the fun aspects. But all of this is subjective and depending on the person. In this case, it very much feels like there is an aspect of a shared living space that creates an assumption that you don't need to schedule time together -- and heck, even monogamous people really miss out on this and their relationships suffer for it.
Because you don't have that sort of structure, of course your brain begins to offload that by trying to manage what goes on between him and C and gets hung up on the fact that C might have come in to try and cowgirl your husband. You're sitting on a foundation that isn't solid and your partner has a tendency to neglect your relationship. It's a perfect recipe for anxiety and fear. And then the fact you're trying to build this relationship with C and ALSO being neglected by her... well, that's the fire in this flambe.
I wrote a polyamory introductory article that talks about thinking of your anchor and also working on figuring out how non-monogamy fits into your life. It doesn't seem like you and your husband have sat down and talked about this. Do you want a triad? What is your ideal? Has your husband addressed the fact that C has plans that seem like their relationship will definitely come to an end (and maybe he doesn't mind)? Figure out what it is you both want, what you can compromise on and what you cannot. And figure out if your relationship is actually a triad or a triangle. A triad being three individuals who see themselves as a collective relationship and a triangle being people who have parallel relationships with one other.
Once you've done that, you'll be in a better position either for you both to confront the dynamic between you and C or for you to do that yourself. Don't allow your brain to fill in the gaps with conspiracy theories about C's intentions because at the end of the day, if your foundation with M is solid and you trust your husband, then C's intentions are her own problem to manage.
It's kind of hard for you to trust M will stick with you if he's not showing you that you matter to him and has a pattern of chronic neglect but... again, as I said, if you don't define what your time is together and what's for others and what time is house management time, it's super easy to neglect someone. Monogamous people do it. It doesn't just happen within polyamory.
Last but not least, it might be worth you working with a polyamory friendly therapist about putting boundaries around C if it turns out you do not want to date her anymore. It might be that once you figure things out and you can trust your partner more, you don't feel so panicked about C and you butt heads less.
But I also think being able to manage a situation like this where maybe you and C don't work out so well and M rightfully so wants to continue dating her, it might make it easier for you to do that if you're able to put down some boundaries. It's not about "veto", but it's about being able to manage the situation. If you think of C as a friend or family M has that you do not get along with, you can use similar strategies to manage that.
You don't sound like you're struggling with a lot of self-attack, but just in case, I want to also say that a lot of the fears and emotions you're experiencing are really normal and to not be hard on yourself about them. This is totally understandable and actually very logical.
I hope this helps and good luck!