I don't agree that the twist at the end ruins it - in fact it's the opposite; instead of being yet another boring song about a how awful it is to break-up it illustrates an interesting point. Until you get to the revelation of his infidelity you think that the song is about how he feels alone and disempowered, with no part to play in her life any more. You sympathise with his self-pity. Then you find out that the reason he is alone, and the reason why he doesn't have any part in her life is because he threw it all away. It's all his fault, he effectively chose it to be this way, and now he's complaining because he is on the sidelines.
I don't agree that the twist at the end ruins it - in fact it's the opposite; instead of being yet another boring song about a how awful it is to break-up it illustrates an interesting point. Until you get to the revelation of his infidelity you think that the song is about how he feels alone and disempowered, with no part to play in her life any more. You sympathise with his self-pity. Then you find out that the reason he is alone, and the reason why he doesn't have any part in her life is because he threw it all away. It's all his fault, he effectively chose it to be this way, and now he's complaining because he is on the sidelines.
That's how it happens in real life, baby.