A 4-1 defeat at Sheffield Wednesday, coupled with results elsewhere, condemned Danny Cowley’s men to finishing 10th in League One this season.
Realistically, their Hillsborough encounter was over at half-time, having trailed 3-1 following poor defensive play.
Despite George Hirst opening the scoring with his 15th of the season, the Owls hit back through Lee Gregory, Saido Berahino and Jordan Storey.
Irrespective of a better second-half display, George Byers’ late header completed the crushing scoreline.
And Robertson criticised his team’s defending in the opening 45 minutes.
The Blues skipper told The News: ‘Obviously it’s a disappointing way to end the season.
‘That first half definitely cost us, when you concede three goals in any half of football you are going to find it difficult to come back.
‘We took the lead and actually played well for the first 10-15 minutes, then the game became stretched for some reason.
‘The gaps were too big, we didn’t match them physically for long periods of the game, the goals we conceded are unlike us, three were from set-pieces.
‘We have got to defend set-pieces better, conceding on three of them is unlike us, we’ve overall been decent on defending them this season.
‘We were better in the second half, we had a few chances, such as Raggs’ header, George Hirst’s one-on-one with the keeper, but it wasn’t enough on the day.’
Pompey’s defeat, coupled with Bolton’s 4-2 win over Fleetwood, meant a day that could have seen Cowley’s men finish eighth, resulted in 10th spot.
And Robertson rued failing to capitalise on the perfect start handed to them by Hirst.
He added: ‘Sheffield Wednesday are a good side, we knew it was going to be difficult, but I felt if we quietened the crowd early on it would help.
‘So to concede three goals before half time is disappointing.
‘There were gaps back to front, we didn’t squeeze up the pitch enough to limit their space, and they have good players in the pockets like Barry Bannan and George Byers.
‘We weren’t at it in the first half and in the second half we were a bit better, but not near our best.’