Please remember... even the most perfect dosage of medication isn't going to be 100% effective 100% of the time, okay? There can be a trigger at any time and her therapies... her coping skills... are either backed up by the meds or they're not. Is it possible that while she was away from home she missed her medication?
She's lucky to know, so young, that she has this diagnosis. So many adults don't have a clue that they have it and need treatment. Look at all of the adult daughters on here that talk about their own mothers and mothers-in-law who have or likely have BPD and wish they were in treatment and would acknowledge the problem. Your daughter is not only decades ahead of them, but a lifetime ahead of them. That's some pretty amazing parenting on your part, okay?
My eldest daughter, who has ADHD and ODD, plus some related anxiety and sensory issues, and is 12 years old has a tendency to be very high on the histrionic scale and border on paranoid when she feels wronged or sad or upset. My middle daughter, who has Autism, ADD, ODD, Anxiety Disorder, Sensory Disorder, can have epic meltdowns that are beyond histrionic. The stress related to their issues can be overwhelming. Like you, I worry about the future and my parenting.
But you know what? You're not starting completely over. You have baby steps and a routine that you must follow in order to get her to step down. You can get your other child/ren into a safe location until that happens. Make sure that she's (back) on her medications and reassure her that she's safe and loved. I have a feeling that the trip may have been very stressful for her, since she was away from her comfort zone for a long time and she probably got overwhelmed. Now you have more of an idea what her tolerance time is to be away from home.
Get her immediately back into her normal daily routines and the things that comfort her. Get her to her therapy.
Baby steps, okay? One step at a time. One minute at a time. Put on your armor.