Pompey Academy exited the Youth Alliance Cup after losing on penalties at Forest Green in the second round.
The home side progressed with a 5-3 win in the shoot-out following a 1-1 draw after 90 minutes in Gloucestershire.
They took the lead midway through the first half, but Gabriel Kamavuako teed-up Adam Payce (pictured) to level before the break.
Assistant professional development phase coach James Barlow said: “We had a really good 20 minutes right at the start and the momentum was in our favour.
“But we didn’t capitalise on that and then it takes just one moment or mistake to change the course of the game.
“Forest Green played out and we didn’t get good enough pressure on the ball. It went over our back-line and they finished well.
"We had a really good 20 minutes right at the start and the momentum was in our favour."
“But there was then some really positive build-up play from us – the best we’ve seen so far, barring the game against Bristol Rovers.
“It went from Toby Steward in goal right the way through and Gabiel cut the ball back for Adam to finish.
“The game was frantic – as cup ties often are because you obviously need to win in order to progress.
“But Forest Green were very physical and posed a threat. We had good chances, though, and had a goal disallowed.
“It went to penalties and from there, it’s down to individual decisions. They slotted all five of their efforts away and unfortunately we went out.”
Pompey: Steward; Kaba, Spurway, Fox; Setters (Howell 64), Simpson, Jewitt-White, Hughes; Payce (Fish 68); Gifford (Bridgman 61), Kamavuako