Tottenham Hotspur player,Harry Kane again scored the only goal as England beat Lithuania 1-0 to end another World Cup qualifying group unbeaten.
“We wanted to score a load of goals and when you don’t, you understand the reaction,” Southgate said.
“We’ve won two matches and after both we’ve come away a bit flat.
England registered four shots on target against a team ranked 120th in the world, with Kane’s penalty taking his tally to 15 goals in his last 10 matches in all competitions for club and country.
England end the group with eight wins and two draws from 10 games, and Southgate tried to find the positives, which included his switch to a three-man defence of Michael Keane, John Stones and Harry Maguire.
Jordan Henderson and midfield partner Harry Winks both played 98 passes as England dominated with 70.5% of possession, but they again struggled to create clear chances
“We’ve not been able to break down a packed defence, but the change of system is a starting point,” he said.
“We’ll get better at that. But there was very little space to play in.
“We played a packed defence who don’t even come out at 1-0 down, and we have to find a way of combating that. We didn’t manage to do that.
“For me, the qualification was done and it was the time to change the system and look at the implementation of that, looking at players who can take us forward.”
‘How can we improve if we don’t play the best?’
Southgate stated pre-match that he was planning to look at a 3-4-3 formation in November’s friendlies against Brazil and Germany, but was able to start a game early thanks to Friday’s qualification for the finals in Russia.
The England manager believes his side will learn more from games against the current world champions as he aims to shape his selection for Russia.
“We’ve got some good attacking players who will score goals, and the challenges of the games going forward will be different,” he said.
“The style of the games, different. We’ll be tested defensively, but there’ll be a bit more space to play in, even though we’ll be playing against a better quality of opposition.
“Our defenders need a different sort of challenge. They need to play out under more pressure, and to do defend against top strikers. That’s part of their learning journey. If we’re going to go with young players, how are they going to improve if they don’t play against the best?