The Bees slumped to a fourth loss in six matches after Francois Zoko's double cancelled out Clayton Donaldson's opener.
And McCormack felt Boro turned the game into a battle - one which he admitted Brentford lost.
"It was a frustrating game," he told the Bees' website.
"We started very well, the mistake for the goal knocked the stuffing out of us and we lost our way.
"We conceded another goal, they got the run of it and they turned it into a battle. We didn't cope too well with it.
"We tried to play football against a team that wanted to beat us up; they coped better with it than we did."