I love this song. To me, it's not so much about losing/leaving someone, as finding hope afterwards. I might be way off base with what I'm going to say next, but that's OK - the beauty of music is that a listener can interpret it in different ways, depending on life experience or current mood.
I think this song is also about two people coming back to each other. It's hard for me to hear this song without hearing two sides to the story. It's both of them that can't get the other out of their veins. They're a part of each other, and no matter how much things change, how hard life gets, or how hard the relationship gets, they'll keep coming back to each other. The last part "you catch a glimpse of sunlight/shining, shining down on your face" gives me hope that these two will realise that the love they have has survived all kinds of change, doubt ("oh you run away/cause I am not what you found") and devastation, and that because of that, there's hope that it might just work out after all.
That's just my two cents. Call me a romantic, but a song like this seems heartbreaking but hopeful and beautiful all at once.