Hi PN, nice to see you here.
I can tell you what I would like to do (I'm not always the best at not just being reactive). If I conciously think about it I sit with what I'm feeling until I get to the root behiend it and can talk about it in a less emotional manner.
I also use more than one method to relay what's going on for me. I'll talk about things and then write about them, either here on in a letter. I express myself better through the written word a lot of the time, especially when it comes to emotionally charged topics.
Sometimes the blowout conversations do happen though. And it's not always a bad thing when they do. I find that sometimes it's the best way to get to what's going on authentically. As long as the conversation (fight) is based around what happened rather than personal attacks it can still be useful. The talking and crying can be very cathartic and in some strange way bring you closer together. (at least that's been my experience).
Some of what happens in poly isn't easy to foresee and as such the first time it happens you might do and say things that you regret. We're all human, we learn from these experiences. When I've said or done something that I regret I look at what was going on for me and I try to articulate how where my boundaries are around the situation.
Everything will be ok in the end. If it's not ok it's not the end.