Question: how to be in a relationship with a borderline?
Answer: In your relationship:
Avoid the temptation to isolate. ... 
You're allowed (and encouraged) to have a life! ... 
Join a support group for BPD family members. ... 
Don't neglect your physical health. ... 
Learn to manage stress. ... 
Listen actively and be sympathetic. ... 
Focus on the emotions, not the words.