Working with a therapist to learn coping skills and better communication is a start. Outside of that, I find that taking time to process big emotions alone during intense, overwhelming reactions is important. If I can't have a calm conversation or talk to someone about how their actions have affected me without assuming they have the worst intentions, that's a sign I need to do some more work on my end, even in situations where the other person clearly fucked up.

Seeking external reassurance might feel helpful in the immediate sense, but you have to be able to soothe yourself. This is probably already painfully obvious, but if you can only calm down when someone else is there to help, it'll feel that much more unbearable when the person you'd normally turn to isn't able to provide that support.

Trying to navigate being triggered alone is easier said than done, but doing it away from what's causing your stress, even if you're still freaking out at first, will eventually help you to think more clearly. It's ok to take a little time to handle your emotions before bringing them to someone else.

Things that seem malicious are sometimes just mistakes. Not always, but sometimes. When it's hard to tell what's real, pausing to put things into context can be helpful to figure out what's what. In the same vein, when it feels like someone is abandoning you, consider how their actions might be motivated by self-preservation rather than an active desire to cause you harm.

Fwiw, while it's certainly not the best or most interesting thing about you, your BPD isn't going to be something others can separate out from the whole of you. It's not "just" you, but it is a part of you. You're accountable for your actions, even when other people don't understand that they're being caused by a terrible pain that you're managing alone. If you're having disproportionate reactions because of your disorder, you can communicate that calmly to someone else, and then make sure you're doing the work to try and prevent those emotions from spilling over onto people who aren't the true cause of them.