snowmonkey,
Welcome and hello

People with BPD tend to externalize their emotions, so it helps when we are centered and grounded. Helps them get centered and stabilize their own feelings when ours are solid. It's why we talk so much here about taking care of ourselves and making that a priority, because it helps us and helps stabilize the relationship.
It sounds like a good time to think about this:
Three weeks ago she creates an argument out of absolutely nothing and causes us to break-up.
She has probably been told her whole life that she is over reacting, creating mountains out of molehills. It's hard to imagine the toll that takes on a person's ability to feel "ok" about themselves.
Which is why you hear so much about validation with BPD. For her, the argument was absolutely about something. She felt invalidated, and went seeking validation elsewhere, immediately. She found it in her colleague and acted impulsively -- she was desperate to feel validated.
It is both easy and hard to practice validation. When she is upset about something, try validating her feelings. "I can see you're upset by this. It hurts when someone doesn't listen to us, I can see that now. Is there something I can do to help here?"
So, easy to find the words, though sometimes hard to say if we're not feeling empathy and compassion -- she is likely extremely hypersensitive to body language, facial expression, tone of voice, in addition to words. This is the hard part, to draw on our own empathy for how someone feels when they are suffering. It might not make you suffer, it definitely makes her suffer.
Is that something you can do with her?