A breakup isn't just physical separation from a person. It's cutting off the opportunity to form new memories together, and to refresh old ones.
Memories are half of who we are, so when we form a lot of them together we literally become a part of one another. When we stop forming them, it changes who we are. It's a cliche to say that a breakup leaves us with a hole—a missing part—and I think that's far less metaphorical than it sounds.
Another cliche is the sudden realization that we can't remember what a past lover's face looks like. This is the most horrific part of all; not only are we missing parts of our identities, but suddenly the parts we kept are degrading with time.
We form new memories, of course. Grow new parts to fill in those holes and replace the fading bits. Yet there's always the nagging question, the creeping horror: are the new ones any better than the old ones were? We can no longer remember.
Maybe that's for the best.
On the bright side, stay tuned.