Marcus Tavernier spoke after Boro's 3-1 defeat away to Swansea City on Saturday.
The midfielder equalised for Boro early in the second half with a powerful first-time effort.
But minutes later, Marcus Browne was dismissed for the Teessiders, Paddy McNair also seeing red as Swansea retook control of the game.
"We knew that one-nil down at half-time the game was still there," Tavernier said.
"In the second half we came out bright and got ourselves back in the game.
"The sending offs are frustrating, we have made those mistakes before and we can't keep doing that."
Watch Marcus Tavernier's full interview in the video player above.