Russell Slade admitted Leyton Orient were "devastated" after missing out on a trip to Wembley in the Johnstone's Paint Trophy.
The O's had overturned a 1-0 first-leg deficit against Southend with goals from Shaun Batt and David Mooney, either side of Barry Corr's strike, leaving the teams tied at 2-2 and heading for penalties.
But a last-gasp Ben Reeves goal secured a 3-2 aggregate win for the League Two Shrimpers and booked a final meeting with Crewe in April.
"They are devastated in the dressing room, it's football and there is only one winner and you never want to be a loser," said Slade.
"It is very hard to take and it will take a while but we need to show some character and bounce back.
"We came up short in the end but I have to say I didn't see that happening.
"We normally defend a lot better than that but we were winning and I thought we were the side that looked like scoring another."