The young Dons fell to their second successive defeat after an 86th minute sucker punch at Northampton.
Tyler Burey had put Wimbledon back on level terms from the penalty spot, but they eventually went down 2-1 in the EFL Youth Alliance, despite another notable performance from promising defender Tom Scott (pic).
"For 65 minutes we were better than Saturday, although we got pulled into a battle far too easily in the last 10 minutes," said Academy Head of Football Mark Robinson.
"However, I cannot ignore the first 20 minutes when we were weak in everything we did and looked frightened to play. After their goal we came to life, but that's easy to do and we need to take our game to teams from the first second.
"It is where we are at the moment though, players are not willing enough to take responsibility on and off the pitch and your performances and results will always reflect your squad mentality - not the few who try and drag the others along with them.
"There have been improvements, but every single one of them needs to be able to take on board information better. We will continue to stretch and challenge them because it has to be about their development and if that's means we continue to lose games - so be it.
"We only won one of our first nine games last season. It has to be about producing better all-round footballers and that includes how they conduct themselves off the pitch.
The Dons started the game poorly and could not find any kind of rhythm. Although Northampton were the better side they only forced goalkeeper Edward Ogbebor-Onaiwu into action once in the first 20 minutes when he made a fine save with his feet.
In the 25th minute Wimbledon conceded from a corner. They'd failed to learn the lesson from Saturday's defeat at Dagenham and Redbridge in their opening game and a far post header. Jay Williams was quickest to react to volley home from five yards.
The Dons responded well and started to pass the ball around. The front three of Bobby Mills, Great Evans and Burey caused the Cobblers all sorts of problems as Ossama Ashley took control in midfield. They fashioned plenty of half chances and got into good positions.
Then, in the 38th minute, Mills found Burey who showed great pace to go round his man only to be dragged down in the penalty area. Burey confidently tucked the penalty away himself.
The Dons started the second half in similar fashion, passing the ball much better. Evans found Mills run in the 55th minute and finished brilliantly, only to find to his frustration he had been flagged for offside. Going into the last ten minutes the game become a real battle and the Dons lost their fluency.
With two minutes left a rare Cobblers attack resulted in a cross partially punched clear by Ogbebor-Onaiwu, who continued to run out the penalty area to try and win the ball.
Northampton made a sideways pass and Mathew Slinn took a touch and delivered an exquisite chip from 25 yards that beat the recovering Dons defenders.
It was harsh on the visitors, based on the last 65 minutes of the game, but it meant they again suffered defeat in just a matter of days.