This is something that a lot of people with BPD suffer from, and it explains so much as to why it has been so challenging for me through my teens and young adult years to maintain relationships/friendships. 
It's essentially the "out of sight, out of mind" concept brought to life. If I am not physically interacting with someone, face-to-face, it is REALLY easy for me to forget about them, and forget any sort of memories with that person. Even their face, voice, what it feels like to be with them, etc. becomes so hard for me to picture in my head. This is what makes it so easy for us to split people.

What makes this doubly challenging for me is that my boyfriend lives two hours away, and we are both fairly busy with our own lives. We only see each other maybe twice a month, three times if we are lucky. I've fortunately found little "anchor thoughts" that help me stay grounded when I feel myself slipping into an anxious state. 

Anyway just thought I'd share that little bit about object constancy. Because when I first read about it (just a couple weeks ago) EVERYTHING I have EVER experienced finally made sense. 