If I blank out the specifics, it becomes:

Before I met [name], there didn't seem to be room in my life for a single additional element—especially not something as significant as a [description]. But he'd slipped into unexplored, empty places when I wasn't looking.

So beautiful. What I really loved about it is the phrase “but he’d slipped into unexplored, empty places when I wasn’t looking.” Fill out the blanks and see what it does for you.