He keeps his promise, at first. Gathering the tufts of the truffula trees manually is slow, but sustainable, and the environment can survive this way, at least.
But the peace is thin, and it is fragile, and it is broken, of all people, by his mother.
He lets his mother talk him into breaking his promise. Thinking back, all he ever wanted was his mother's approval - it was hardly about the rest of his family.
The Lorax calls him better than this - better than cutting down the trees, he assumes, though he suspects there is something more to it than that. Better than breaking his promise, he thinks. (The Once-ler pushes this one to the back of his mind. He'd rather not remember the exact moment he tossed away his morals for his mother's approval.)
He certainly was better than this, the Once-ler thinks. Some part of him is saying that he should listen to the Lorax. It was a part of him that he had hardly ever listened to, so he didn't listen to it now. It also happened to be the part of him that wouldn't let him pretend his life was fine. That he was okay.
But the trees are hardly falling like flies, and there were still miles of Truffula Valley to cover - it would take ages for them to cut all the trees down.
Two years, as it would turn out, is not ages to ruin the environment. It does not take them ages to ravage the land and keep on biggering and biggering and biggering. It doesn't take ages for the thneed to take the rest of the world by storm, for the Once-ler to abandon who he had previously been and step into this new skin made of fine, glittering things.
Here, our Once-ler is twenty one, hardly our Once-ler anymore, blinded by success and ambition and maybe even the want for his family to care about him.
He takes to wearing sunglasses despite there being no sun and no light but the one from the thneed factory - tinting the smog blue so that perhaps he could pretend that his biggering wasn't harming a thing. The suit he wears is a deep, rich green, and Norma accuses him of choosing green because it reminded him of money. (She is wrong in this respect, though the Once-ler doesn't tell her this. The suit is green because the part of him that won't let him pretend remembers when the Truffula Valley had teemed with life. When he had pretended to be irritated with the animals practically living in his makeshift house. When he was starting to be alright, now that he was away from his family.
When he had been happy.)
The animals have all but stopped visiting since the thneed took off. Even Melvin has stopped listening to his ramblings, and he doesn't know what the mule is doing anymore, but Melvin sure isn't sticking around him.
(He doesn't miss it. He doesn't. He's fine.)
Two years was not ages. And it certainly took as long to get to the very last truffula tree.
The felling of the last tree happens almost in slow motion, landing with a thud that shakes the earth beneath their feet. Maybe that'll stop you, the Lorax says morosely, drooping underneath the weight of all those trees that had fallen.
The Lorax sends the animals to find a new home, one that was unpolluted and safe. And the Once-ler...
He tries keeping them there despite knowing they couldn't stay. He pretends that they'll find a new home somewhere, safe from him. He hopes.
The Lorax leaves, too, lifting himself by the seat of his pants into the smog filled sky, leaving behind nothing but the stone-cut word unless.
And so the Once-ler is left behind in the wasteland he had created, a constant reminder of the mistakes he had made. The thneed, too, is a harsh reminder of those, and he thinks that he should have seen this coming, that he should have listened to the part of him that wouldn't let him pretend.
Be careful which way you lean, the Lorax had told him, and he understands it now - he had flown too close to the sun.
Here, the Once-ler is twenty one and left hopeless and helpless and broken beyond belief: manipulated and abandoned by his family. Ruined by his own mistakes. Left behind by what had become a makeshift family for a makeshift boy that no longer existed. (His old self is someone he could never return to. Redemption is an option that is decades away.)
He is angry, bitter, resentful - the why is self-explanatory.
There is no one left to fool but himself. He cannot even pretend this away, the dust and destruction and the dark, dark sky.
Pretending never works again.