Anyway, could it be that feeling romantic love towards someone serves as a trigger that brings out this BPD side of a person that they've managed to keep in control for a few years?
If I'm the trigger for her, can there ever be a chance for us to be friends again?
Yes, I think some people with untreated BPD are heavily triggered by feelings of romantic love. This has to do with the reality that genuine romantic love, by its very nature, is mutually selfless. It is counter to everything the motivates a person with BPD.
They usually suffer from depression, and tend to have very low self-esteem - if not downright self-hatred. They craft themselves into the "perfect" friend/lover in order to attract someone's attention because they believe that is what they must do since they do not believe who they really are is someone worth being loved - if they even know who they really are. They don't do this with the mental intention of doing so, not really. It "just happens".
They've learned, over time, how to garner positive responses - how to mirror our likes and dislikes, our interests and our passions, so that we will invest in them. But it's all a lie - always. They will change, like a chameleon, for the next friend or lover, and for them it is an exhausting effort to keep any friendship or relationship alive.
It is so heartbreaking how desperately they seek to be loved and adored yet sacrifice the genuine love and adoration they could have had for something false and temporary. They never really get what they want, in the end, and they are blind to this self-deprivation cycle.
You did nothing wrong in loving your ex. But she cannot ever actually love you back, or anyone, in any real way until she has recovered from BPD through therapy. I think it is possible she could be friends with you, but that depends on whether or not she can live with knowing that you know she's a fraud.
In my experience, they are unable and unwilling to give any real effort to anyone who has learned their truth.
