Interpretations:This Is Only Going To Go One Way
Attitude based on determinism
The song's title is a saying typically used to express dread that a situation will turn out disastrous. But I think the song subverts this by taking the saying in a more literal sense, that the outcome of everything is already fixed in advance--philosophical determinism, if you will. That's suggested most clearly by the line at the end: "Brace yourself, your destiny's written." The lyrics also contain reminders that death is where we're all headed.
However, the tone is not purely dark. The lyrics seem to teach that in response to this determinism, one should take a carefree attitude and live with gusto. The bouncy sound with the toy piano supports that. Verse 1a: don't try to re-live the past; time only flows in one direction. Verse 1b: If you're faced with a difficult choice between two alternatives, you might as well just flip a coin, because anyway what will happen will happen and you can't predict it. Verse 2: Love the one you've got, though it may seem stormy at times. Verse 3: Death is inevitable, we have no real control, so just charge ahead. Geronimo!