PRINCETON, NJ — New Jersey Gov. Chris Christie says he underwent secret weight-loss surgery because he had been feeling frustrated and angry over his inability to slim down.

The governor told people attending an event at a Princeton bookstore Tuesday that he got upset with himself when his clothing wouldn’t fit.

Christie appeared at the shop with MSNBC hosts Joe Scarborough and Mika Brzezinski to promote Brzezinski’s new book “Obsessed,” which includes an interview with Christie about his weight.

The governor said he often hears from people who don’t understand the struggle.

“It’s not as simple as, ‘Push yourself away from the table and you’ll be fine,’ ” he said.

The Post revealed earlier this month that Christie underwent lap-band surgery in February.