In 2019, a police officer fired rubber bullets toward a psychology student named Gustavo Gatica, just one of the thousands of protesters demonstrating across Chile against the nation’s government and deep inequality. Mr. Gatica lost one eye and was blinded in the other.

Mr. Gatica considered it a devastating sacrifice, but not one made in vain. The protests forced a process to scrap the Chilean Constitution, which still had roots in the nation’s bloody 17-year military dictatorship, and write a national charter from scratch. Mr. Gatica became part of a national campaign for a new, hopeful path forward for this South American nation of 19 million.

Now, four years later, after a series of bruising political battles and votes in constitutional assemblies and on drafts, Mr. Gatica finds himself in a disorienting position. On Sunday, he is planning to vote to keep the dictatorship-era Constitution that he lost his vision fighting to replace.

The reason? The proposed charter Chileans are deciding on would actually pull the nation more to the right.

“Unexpectedly, they managed to write an even worse constitution,” said Mr. Gatica, 26, sitting in the psychology practice he started in Santiago, Chile’s capital, a few blocks from where he was blinded. “In 2019, I never would have thought we’d be at this point.”