England captain Chris Robshaw said immediately after the game that his side had "unravelled" as Wales stormed to a record 30-3 victory to win the RBS 6 Nations title.
And 48 hours on, Lancaster said: "When we made the transition from the 2011 World Cup squad through to where we are now, we wanted to find out more about the players and we did that, particularly in the last game. We were tested at the highest level. The final 20 minutes was the most disappointing thing about the game, to let it slip away like that."
He went on: "At 9-3 we were still in the game. We put the ball straight into touch and had a period of defence which resulted in 12-3.
"There was then a turnover at the breakdown - which we still need clarification on - and that was the tipping point in the game (because it led to Wales' first try).
"Whatever happened in that breakdown it was disappointing to lose composure and let the game slip away.
"Our ability to retain our composure in the heat of battle (is one of the biggest lessons to take from Cardiff)."
The other is execution. Manu Tuilagi should have scored in the opening minutes but he knocked the ball on and too often England forced offloads or lost the ball in contact.
"Everything matters in the game, whether it is the first minute or 21st minute, you have to take your opportunities when they are presented," Lancaster said.
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