The year before his temporary switch to the Blues, the 23-year-old failed to score in 31 Championship appearances for Rotherham.
Perhaps his barren spell for the Millers hampered his start in royal blue, with the striker often a figure for ridicule during a 13-game goalless run.
And the Foxes forward explained he’s never felt better in the final third, as a result
Hirst told The News: ‘It’s amazing what a little bit of confidence can do for you at times, it’s something which gets overlooked, you can have the best player in the world but if he's got no confidence he’s not going to play as well as the worst player in the world who has bags of confidence.
‘It’s a mental thing, sometimes you can’t really control it, you float in and out of it, especially as a striker when you are hitting the back of the net on a regular basis that confidence comes streaming through your body so it’s hard to explain where it comes from why it comes and at times why it’s not there.
‘My confidence was at an all-time low this time last year - now it’s at an all-time high.
‘It was always a case of once I got into the team, I felt I could stay there and got until the end of the season and look back and it couldn’t have gone much better.’
And Hirst has described why he felt at home by the end of his temporary spell on the south coast.
He added: ‘I can’t speak highly enough of my time at Pompey.
‘I had a tough year last year at Rotherham and didn’t get as many games as I would have liked and for me I needed to come to somewhere where I was going to feel loved and able to show what I can do.
‘For me it was a tough start at Pompey and not playing a great deal. Once I got into the team I felt that was where I belonged.’