I edited some of your words for clarity.
...I missed Arthur and since he kept texting me (which only made me miss him worse) I asked him to go mute for the rest of saturday and sunday... I just felt not having to check and text back would give me some more space to breathe and enjoy my time without him.
Nothing wrong with that. When I am apart from my nesting partner, we often go a day or three without texting if either of us is very busy, either with family, friends or dates. Maybe a quick good night text is sufficient, if that.
Arthur had failed to communicate his schedule for the next week (even though I had asked several times). I had started making plans myself, and found myself knowing that I would not be able to see him Monday evening and invited to a date on Tuesday.
I'm not sure who you can or can't see Monday or Tuesday here.
I have been on a few dates with another man. Arthur knows of him, and has asked me to disclose to him when “something happens." [sex]
This time I was invited to OtherDateGuy's home rather far [away], with an implicit invite to stay the night... So, idiot that I am (and thinking that it would be helpful for him to know as soon as possible so he could fit it in his schedule), I texted Arthur to ask him if we could meet for lunch on Monday or Tuesday to talk “ground rules"-- [that is:] communication around overnight stays [with others] before I had the date [with OtherDateGuy] on Tuesday.
I do not see ANYWHERE in this quoted bit that you made a "big mistake" or did something "idiotic"! Why escalate a perfectly reasonable thing you wanted to talk about, to make things open and aboveboard in YOUR relationship with Arthur (and a possible new partner), into something YOU did to damage the already very troubled relationship Arthur has with Brianna?
You need to separate things out. You are way too entangled in what's going on, on the other side of your V!
The fights and talks, or lack of talks, and flounces out the door that A and B are doing, have NOTHING to do with your actions, or presence, or decisions.
What I didn’t know was that Arthur and Brianna again had a rather challenging weekend and had decided to talk Sunday night – yet not had that talk. When Arthur got my text he got antsy, and Brianna noticed, and asked to have the talk.
I don't know why you need to know anything about their talks. They don't concern you.
I don’t know what exactly happened, but must have been terrible: hours after my text... Arthur messaged me to come meet him, answering my, “Is everything okay?” with a, “Nothing is."
Drama! "Nothing" is all right with A and B. That doesn't mean "nothing" is right entirely, or between you and Arthur. He is confusing [A+B] with [A+you].
I was sent into a half panic when suddenly Brianna texted me too, to go see Arthur right away. I was terrified! I thought that I had fucked up and he would just end it, or worse, my text might have prompted him to end it with her (the thought made me feel terrible).
You seem to think you have a lot of power over what goes on between A and B!! You want to discuss boundaries around a sleep over with another person, and somehow you extrapolate that would affect the A and B relationship? Why would you even think that?
Please learn to separate things. Polyamory requires a certain amount of privacy and independence between the various Vs going on.
...I found him in tears. Brianna had left (a day early). He told me he had fucked everything up (with Brianna), and hurt her so bad.
...I was just confused as hell. Long story short: nothing has changed. They had a fight (and it must have been bad), but they decided they wanna fight for their relationship.
So you were confused because you thought that your request to talk about a sleepover with OtherDateGuy had caused a fight between Arthur and Brianna? How does that even follow? Why are you so quick to take responsibility for other people's problems? You won't be able to do poly enjoyably until you let go of your partner's issues with his other partner(s).
Texting me must have been a moment of real desperation for Brianna, and it felt like her extending an olive branch, which I have decided to grasp...
Olive branch? Her wanting to tell you what she fights about with Arthur is a gesture of peace to you... how? Isn't it just a burden on you about something that doesn't concern you?
Brianna and I had a (really) brief, yet somehow impactful chat, which ended with me offering to chat more after she and Arthur talked. Today (three days later) I have decided to reach out again, asked how she was doing.
Why are you taking on the burden of counseling your metamour?
I told her I see that she and Arthur want all three of us to meet, but that I have problems understanding the ‘why’.
Maybe consider why YOU would even want to entertain the role of counseling THEM about THEIR relationship. Who cares why your meta wants you to counsel her? That's just way too entangled for someone you're not even friends with. You've only met her once, for an hour. She's dating your bf. She wants you to help her negotiate her relationship with her bf (whom you happen to share).
Honey, that is not your role. You're too close to the whole thing. Brianna can talk to other friends about her mess. So can Arthur.
Once again, I'd consider stepping back from Arthur until if and when he and Brianna sort things out, on their own, with help from other friends, or a counselor. It sounds yucky, and gross, and exhausting for you... and almost like triangulation. Playing you and Brianna against each other.
I’m trying to focus on my relationship [with Arthur] (admitting that it had been challenging for me, too) and don’t see how crossing over and getting involved in whatever struggles they are facing could help de-escalate the situation. I told her I’m having a hard time grasping at what exactly it is Arthur wants us to talk about.
I really really think the above is something to tell Arthur, not Brianna. ie:
"Arthur, I don't want to hear anything more about the fights between you and Brianna. And I'm going to step back from you for X amount of days or weeks. Because you can't focus on me right now while you and Brianna's relationship is in so much trouble."
I told her that if there is anything she’d like to speak about, that she thinks might be getting lost in translation, she could always let me know.
No. Not your role. You have a leaky hinge (Arthur). He wants you ladies to talk to each other and sort things out, since he seems incapable of doing it. That makes for a bad hinge. I'd lose respect for a partner who can't keep a healthy boundary between his 2 separate relationships. I'd step back. Firmly. Someone's got to be firm!
What are you getting out of being close to a partner who is having so much trouble in another relationship and sharing the gory details? Why do you think you need to counsel your meta? Is it satisfying to you somehow? Do you think this is how poly is "supposed" to work?
I know that is probably going against everything that you guys would advise me to do, but it can’t go on like it is and I feel like I’m gonna have to give it one last good shot – especially since the drama on Sunday feels like it was at least partially my fault.
It wasn't. I hear you feel it was, but it really wasn't. The only thing your fault is you thinking something that needed to be discussed between you and Arthur about overnights with others, could cause a fight between him and your Brianna. You seem to have a too ready guilt complex. Where does this come from?
I have cancelled the date, and told OtherDateGuy I’d need time to sort out my relationship [with Arthur] first before I could focus on something new. When he offered to basically wait, I told him not to, it felt unfair.
Unfair to OtherDateGuy? Why? You may indeed see changes in your self awareness about how to do poly with Arthur in the coming weeks, pulling back while he sorts things out with Brianna, etc. Maybe you could have a fun date or 2 with OtherDate while Arthur is having all his drama with Brianna...
Or later, if and when Arthur calms the fuck down and is ready to be a better bf to you.
Arthur doesn’t think it’s the right call, and feels guilty. But to be honest, I just feel like I don’t have the emotional energy to establish a new, meaningful relationship at this point. I like OtherDate, but I felt nowhere as strongly about him as I do about Arthur, so it feels like I made the right call.
There is no need to "establish a meaningful relationship" with OtherDateGuy or anyone else right now. Why so serious with ALL of this? The beginnings of any new relationship, or sexy fun friendship, or whatever, with anyone, should be easy and enjoyable. You're foundering in the drama between A and B. You're also dwelling on "establishing a meaningful relationship" with the other guy, whom you barely know. It sounds exhausting.
Take it easier on yourself.
Please don’t behead me for this
There's a lot of hyperbole in your post. Take a breath. Learn to see where your responsibilities lie, and WHERE THEY END.