Plagued by injuries over the past few years, critics began asking if he was good enough and he suffered self-doubt which increased the more he held the feelings in.
"You start to read into things like that," he said.
"A combination of things led to me thinking 'Why don't I just quit now?'"
Carter, who was born in Southbridge, a small town southwest of Christchurch, said that growing up in a country town and talking about feelings in public wasn't done.
Two years ago, his relationship with coach Steve Hansen had deteriorated and they weren't speaking.
"He just assumed everything was okay and I put on a front."
The team's psychologist helped lift him from his low point.
"I remember when I first started, the team would have a psychologist but it would almost be frowned upon to go and see him. It's like that crazy theory of you're absolutely mad if you go to see a shrink. But the way the game's evolved, you're absolutely crazy if you're not using the psychologist."
On the Rugby World Cup final, left-footed Carter said he'd talked to teammate Aaron Smith about how great it would be to kick a conversion with his right foot.
"He was like 'You'll never do it'. I was like 'man, imagine if we're in the final, we're ahead by more than a try, it's the last play of the game, I did it with my right, that'd be awesome'."
• For exclusive extracts from Carter's book - including revelations about how bad his Rugby World Cup leg injury really was - see tomorrow's Weekend Herald and the Herald Online.
