Cooper: Shootout defeat cruel
Mark Cooper felt Swindon merited victory after seeing them lose to Peterborough in the JPT Southern Area final.
Monday's second leg at the County Ground ended 1-1 on the night for a 3-3 aggregate stalemate, and Tommy Rowe scored the crucial spot-kick in a 4-3 penalty-shootout win as Peterborough sealed a trip to Wembley after Massimo Luongo and Tijane Reis had both blazed over for Swindon.
Cooper said: "We played well and did enough to win the game, we made one mistake and it led to a goal.
"My players gave everything and were deservedly in front at half-time but it was not to be. If you give Assombalonga one chance he will score.
"We played well and fought and you have to hope luck goes you way but it didn't.
"We have got a lot of disappointed young men in the changing room and it my job to get them up for a tough game at Leyton Orient on Saturday."
Cooper also revealed striker Nile Ranger will be out for up to 12 weeks with a hamstring injury.
Swindon were denied a penalty as Craig Alcock's fail on Dany N'Guessan went unpunished but the Robins took the lead 13 minutes before half-time after some comical Posh defending.
Luongo whipped in a teasing left wing cross and Alcock smashed a clearance against N'Guessan before Alex Pritchard tucked home the loose ball.
N'Guessan hit a post before Assombalonga drew Posh level 15 minutes from time with his 24th goal of the season, coolly heading Michael Bostwick's cross beyond Wes Foderingham.
Luongo set the tone for a miserable shootout for Swindon by smashing the opening spot-kick into the stands.
Foderingham gave Mark Cooper's side hope by saving Danny Swanson's effort only for substitute Reis to also miss the target.