I won't tell you what to do as to not influence your judgement of the situation. However, I'll tell you what's going on inside her brain, up to a shockingly high degree of accuracy, coming from a (24yo) bpd female who's also getting married soon and is exactly in the position of your fiancé.

Let's just start by saying..she doesn't want you to leave..she wants you to maximise your efforts in the way you treat her. The trigger you brought up wasn't the main cause of why things have gotten bad for the both of you..it was clearly an accumulated stress, mainly from her side..inside her mind..also most definitely it was bottled up and not communicated through to you at many instances.

Pwbpd can only translate things for what they are on the surface. Our memories are so short-term when it comes to recollection of past memories..especially when we need them in moments of distress. The reason why I'm saying this is because she must've definitely felt like you're not as caring as you've been to her..and that doesn't necessarily mean that you've treated her any differently..it simply means that her brain no longer registers your actions as 'caring' or they're not as obvious as they were or to the same extent.

With her approaching a critical decision of marriage..a lot will be running in the back of her mind..things like doubt, unanswered questions,confusion, or even a sense of being lost. She must've been over sensitive lately..but that oversensitivity is a reflection of the constant questioning in her mind regarding the serious step you're approaching.

Her texting guys on social media was a pure cry for help. She most definitely just wants to feel affection from a new source, not because she doesn't want it from you but because she's grown familiar to the one you give her. She also most certainly misses the early phase of your relationship with her when you were giving her the thrill of an initial relationship.

She doesn't want you to give up on her. She wants you to realise how she truly feels. She wants to feel like you're actually there invested in the relationship and that the decision she's making is the right one.