Mark Kelly blamed a lack of discipline for Pompey Academy’s heavy FA Youth Cup defeat at Cheltenham.
The young Blues appeared to be cruising as they eased into a 3-1 advantage, only to end up losing 8-4 on a crazy evening.
It means they miss out on a glamour third round tie with Arsenal, who will now head to Whaddon Road instead of Fratton Park.
And Kelly knows his players should have shown more control to see the game out from a dominant position.
Pompey’s academy managed said: “It’s funny how momentum can shift because we were in control of the game at 3-1 and looking like we could go on to score a few more.
“But then the concentration levels slipped before half-time and we started to do some crazy things to gift them goals.
"The boys had their heads down afterwards because they know they should have grabbed hold of the game."
“It turned the contest upside down and from that point it was hard to get things back on track. It’s a real lesson for us.
“We did everything right in the first half-hour and looked comfortable, moving the ball around well and creating a lot of chances.
“But you can’t afford to do what we did in that mad 10-minute spell before the break and it came back to bite us.
“Cheltenham have a certain style that suits them, with some powerful players up front. It caused us some problems, while we also caused plenty of our own.
“There were chances at both ends in the second half, but the control and discipline had gone by that point.
“The boys had their heads down afterwards because they know they should have grabbed hold of the game.
“That’s the learning curve with kids and there was no ranting or raving – it’s just about them understanding how to keep control.”