Cracks

I like to think that "Leave the past behind" means to say read the lyrics backwards. Looking back. So the cracks begin to show, your heart breaks, when it's over, just walk away and leave the past behind.

This song suggests heartbreak, but seems to be touching upon something much much deeper and broad in scope - impermanence. Love, as much as we like to think that it is forever, often results in hurt and dusappointment because we expect it to be something that it is not. Like everything in the universe, it is in constant flux, and when it changes/undergoes a transformation, the reality that we have tried to remain ignorant of is right there staring us in the face, resulting in our naive expectations crumbling. Nothing lasts forever, and the only constant universe is change - this is something that we know to be true, but still try to hide from and say there are exceptions to. Rather than suffering when the inevitable change sets in, let go (walk away) rather than clinging on in ignorance. The past is gone, and nothing is the same as it used to be. Accept this, and look at the reality of the present. This is the only way to effectively stop the pain.

it's about being broken so many times and no matter what, you can't not let it show

I think of the cracks as being in the thing that is left. Specifically, I think the idea is that when metaphorical cracks appear in a relationship, the song advises the listener to walk away. So once love stops being perfect, split.