I need to read more but it seems the only way to go about it without addressing BPD is to locate the triggers and then working on descalating methods?
Bingo. Learning communication and conflict-resolution techniques can go a long way to improving a relationship. Basically, sometimes to make things better, you just have to learn how to stop making it worse. I'm not saying any of this is your fault. Far from it. But one thing I've learned here is that things I was doing/saying were inadvertently amping up the drama and leading to more conflict. When I changed how I communicated, things actually improved. If they're not getting help, there are still things you can do. Ultimately, all you can control is yourself and your reactions.
Sounds like you're running into the abandonment fear (as you said) and the feelings = facts phenomenon. That's common in BPD. Their feelings are so strong and they have such trouble dealing with them (BPD is a disorder of emotional regulation) that they'll manipulate facts to fit whatever they're feeling in order to try to make sense of things or to make themselves feel better. It can be bewildering to deal with.
On boundaries, just for example, what's a boundary you want to set that you don't know how to go about setting?
Could you perhaps describe a recent event? Give a he said-she said play-by-play?
I ask because one thing that's so common here is JADE-ing (Justify Argue Defend Explain). It's natural to do. I did it all the time. But, with BPD especially, it tends to just pour fuel on the fire. So much better to use SET (Support Empathy Truth).
For a start, here are a couple of workshops and articles that might be helpful to you:
SET:
https://bpdfamily.com/message_board/index.php?topic=143695.0JADE:
https://bpdfamily.com/message_board/index.php?topic=139972.0Resolving conflict:
https://bpdfamily.com/content/ending-conflictTake a look and let us know what you think! We have a lot more but I don't want to overwhelm you.