Aston Villa were beaten 4-0 by Manchester City last weekend.
Villa were able to hold City off throughout the first 45 minutes at the Etihad Stadium on Saturday, and they went into the break with the score tied at 0-0.
The Claret and Blues were strong defensively during the first half, but after City took the lead in the 48th minute through Yaya Toure the floodgates opened.
Sergio Aguero scored another two goals for City, before Raheem Sterling sealed Villa’s humiliation.
It was another example of Garde’s side’s worrying habit of collapsing after letting in one goal.
And Garde said: “We wanted to go very far in that game without conceding one. The first 45 minutes we did well showing a lot of concentration and dedication to the game. The second half was completely different.
"It's something I can't explain. The mentality of the players has been damaged a lot from the beginning of the season and we are getting too fragile, too quickly in the game.
"It's difficult to be 100 per cent sure but when you see the difference in the first half and the second half there is a deep mental problem.”
Villa are now winless in 14 Premier League away matches, and they appear destined to be relegated.
The Midlands side are currently propping up the top flight standings, and they are nine points adrift of safety.