Halfpenny had been presented with a 55-metre penalty shot with the last play of the game to potentially clinch the series for the Lions in the second Test. His kick fell short and the Qantas Wallabies lived to fight another day in Sydney after squaring the series with a 16-15 win.
That miss preyed on the mind of the 24-year-old perfectionist, although nobody blamed him for missing from such long range having played a full 80 minutes of full-on Test rugby.
"It was hard after the game and I was pretty down. It was tough, really tough, because I had the chance to win it for the Lions and I didn't take it," admitted Halfpenny.
"That's what all the hard work is for, for moments like that. I was devastated at not being able to do it.
"It was touch and go that kick - I felt that if I connected with it I'd make it. But I didn't connect and it took me a few days to shake it off - it was really tough.
"But we had another chance and people made that clear to me. Not often in sport do you get a second chance.
"I had to put it behind me, but I also used the experience. I didn't want to feel like that again so I used it as a motivation because the disappointment can stay with you.
"I wanted to be a part of a Lions series win and it helped me tonight. I've worked harder and harder in training and tonight it all paid off."
What a difference a week can make. After the misery of Melbourne, Halfpenny was walking on air in Sydney as he hit the mark with eight out of nine kicks to end the tour with 114 points, a record 49 of which came in the three-match Test series.
"The contrast in dressing rooms after games can always be extreme. Last week was as low as I ever want to feel, but this is just incredible - the feeling is unbelievable," he added.
"As a kid you have a dream of pulling on a Lions jersey. But I never dreamt of anything like this - to play like that and win like that is something that will live with me forever.
"It's amazing to be a part of a Lions Test series win. It's why you work so hard, for moments like tonight. We played so well tonight but we always felt we had that kind of performance in us.
"We have supported each other as a squad from the start and we've all worked hard. We are delighted with this, we've made history."