"A person with BPD is highly sensitive to abandonment and being alone, which brings about intense feelings of anger, fear, suicidal thoughts and self-harm, and very impulsive decisions. When something happens in a relationship that makes them feel abandoned, criticized, or rejected, their symptoms are expressed." That is the definition of BPD. It's a really hard thing to explain especially when everyone has different symptoms. I had to explain this to a few family members a while back and I started out with talking about my extreme highs which turn into mania, and my extreme lows which some times resulted in self harm. I explain how hard doing daily tasks that seems simple to a regular person, are very hard and seem almost impossible most days. I also talked about how it made my relationship very difficult. Just be honest and explain how it effects your life.