I'm not really following why you are at risk of hurting their relationship. She ended things with you, even though you would prefer that things continue. You are not being irrational and acting out at either of them, or being difficult in any other way. By not giving you and her husband alone time and space, she put HIM in the awkward position of feeling like he was having to be repairman to two anxious women. I don't see where any of that is your fault, nor your responsibility to fix. Simply existing, and being involved with her husband, does not equate to putting their relationship at risk. If they are both down with the basic premises of polyamory, where's the problem?
They were initially looking for a triad, not a V, so even though they are "down with the basic premises of polyamory," the new dynamic might not work for them and I can understand that. I feel a little weird about it too, although I'm not sure why. I guess in a way it feels like instead of all three of us growing closer together, now it feels like M and I are pulling D in opposite directions. Maybe it won't always be like that, maybe it was just the way things were this weekend, but if it stays like that it won't be workable because it'll be too hard on all of us. I never wanted it to be like that to begin with, but when she and I both needed him, she tried to pull away from him and let him give me what I needed. Now that I know she did that, I feel guilty because we agreed a while back that our marriages were the priority relationships (they don't like to use primary/secondary language, but that's basically what it boils down to), and I feel like if he couldn't meet both our needs, his obligation was to meet hers before mine, and he apparently didn't do that. However it does kinda bug me that neither of them told me what her needs were until after the trip was over and it was too late for me to do anything, like insist that he put her needs first, which I think I would've done if I'd known.
She didn't really have much of a choice to give us alone time and space this weekend, because we were all on the trip together. However she did try and give us some alone time, like when she stayed in the hotel room and we went down to the pool and hot tub. It seems, though, that she needed more alone time with him than she got, although she did get some too. He says she was purposely avoiding him (not talking to him or being affectionate with him as much as she would have otherwise) because she could tell being around her was upsetting me. I feel that is not entirely my fault, though, because they both failed to communicate their needs to me in that regard.
As for me acting out or being irrational, I guess they felt that my being as emotional as I was over the breakup was negative behavior on my part. I don't feel that I could've controlled it any better than I did, but if I had realized how bad it was going to be, I would've chosen to not go on the trip at all. But there's really only one thing I think may have been irrational or acting out on my part: at the hotel on Saturday, D and I were taking a shower together (first time we've done that, and actually the first time we've seen each other naked, so for me it was kind of a special and intimate moment). I had finally managed to stop thinking about M for a few minutes and feel less sad, and then just a couple minutes into the shower, D said something about needing to spend some alone time with M afterwards, and just hearing her name brought all my feelings about the breakup back instantly and it hit me pretty hard. I didn't mind him telling me he wanted to spend some time alone with her - I'm glad he did, actually, because that was the only time during the trip they communicated that need to me - but I felt the timing was really bad and somewhat inconsiderate of my needs to not think about her breaking up with me for a while and to enjoy the intimate moment with him. I just wished he could've waited until we finished showering, you know? But my reaction was apparently upsetting to him because after that he felt he couldn't mention her without upsetting me. So I don't know if I was being irrational or acting out there. I understand that maybe my reaction made him feel like he couldn't communicate their needs to me without upsetting me, but really it wasn't that, it was just the timing was bad, and I feel like he still doesn't understand that.
Sorry so long! I'm rubbish at summing things up briefly. ^^; But I'm really glad I'm getting the chance to write all this out and get feedback on it! For me, that's probably the best way to process my thoughts and feelings. So I think this is really helping.